I felt the urge to open this topic, not in defense of Sir Ridley Scott, because he doesn't need any, but to make things clearer, after seeing that more and more people question Ridley Scott's huge contribution for the Alien franchise, by stating that he is merely the director.

While not minimizing the rest of the people involved their merits, we have to acknowledge, once and for all, that, without Ridley Scott, the Alien we all love would have been a totally different film. I summarized below some relevant facts about Ridley Scott's contribution during the production of Alien (big thanks to Strange Shapes, where I found most of the info compiled below).

Firstly, on the story, without Ridley Scott we would not have today the xenomorph of an alien origin, but created by the Company - it remains to be seen after Alien Covenant if David created the xenomorph or just his own variant (excerpts from Strange Shapes):

"Here is a breakdown of the two plots. Giler and Hill’s version is a summary of their script before O’Bannon and Shusett urged Ridley Scott to have the script revised.

Dan O’Bannon’s Alien, a synopsis: the crew of the commercial vehicle ‘Snark’ awaken from cryosleep on a return voyage to Earth. Their ship’s computer has detected an SOS beacon of unknown origin emanating from a nearby planetoid. The crew land, and find a derelict spacecraft containing the corpse of a dead alien pilot. Nearby they find another structure; an ancient pyramid, containing mysterious spore. One of them is attacked and impregnated; the creature erupts during a meal after the ship has continued its journey to Earth. The crew are picked off one by one until only Roby survives, along with the ship’s cat. The Alien is ejected from the emergency shuttle and vapourised. The Snark itself is destroyed. Roby enters cryosleep for the journey home.

Walter Hill & David Giler’s Alien, a synopsis: the crew of the commercial vehicle ‘Nostromo’ awaken from cryosleep on a return voyage to Earth. Their ship’s computer has detected an SOS beacon of unknown origin emanating from a nearby planetoid. The crew land, and find a derelict spacecraft containing the corpse of a dead human pilot. Nearby they find another structure; a concrete Cylinder, containing mysterious spore. One of them is attacked and impregnated; the creature erupts during a meal after the ship has continued its journey to Earth. The crew are picked off one by one, and the Science Officer Ash is revealed to be a Company robot. Ash reveals that the crew were led to the Cylinder deliberately, to serve as test subjects for the weapons division – the Alien is one of the Company’s bioweapons. In the end, only Ripley survives, along with the ship’s cat. The Alien is ejected from the emergency shuttle and vapourised. The Nostromo itself is destroyed. Ripley enters cryosleep for the journey home.

As already pointed out, O’Bannon and Shusett intervened to have Hill and Giler’s draft rewritten to incorporate the alien elements that they had excised. “Ridley read [the original script] and went, ‘Oh yes. We have to go back to the first way. Definitely.'” Though Giler and Hill acquiesced to Scott’s demand, they still managed to infuse the script with the paranoia of a Big Brother corporate entity whose sheer size and oversight leads to the deaths of its employees in some dark corner of space."

Secondly, on the creatures, although O'Bannon introduced Giger to the producers, the key to hiring Giger was Ridley Scott, as O'Bannon himself testifies (thanks to Strange Shapes again):

"The only problem with Giger was the reluctance of Twentieth Century Fox and the film’s producers to hire him. Giger had initially worked on a commission from O’Bannon and Shusett, and though he had begun preliminary design work, he had yet to be brought onto the film in an official capacity. “This man is sick,” producer Gordon Carroll is quoted as saying, having seen Giger’s designs. “I fought a year with Fox to hire Giger,” O’Bannon told Science Fiction Film Making magazine in the 1980’s. “I wrote the script so Giger could design those things, and then they picked up the script and said, ‘Naw, we don’t want this guy. When has he ever designed a movie?’”

“I had a heck of a time trying to get the producers to hire Giger,” said O’Bannon elsewhere. “They really didn’t want to get him involved because he’s not a movie professional; he was some ‘whing-ding’ in Zurich. They wanted to find somebody who had done this before, that they could count on.” Ron Shusett chips in: “The studio let us hire Cobb, because he was more normal. Giger, they were terrified of. They said, ‘these drawings are repulsive – people will stay away in droves.’ For eight months they refused to hire him.”

The key to hiring Giger was Ridley Scott, as O’Bannon explains: “When Ridley came to the project, Ronnie was rushing up with the original draft of the script [and] I was rushing up with copies of Giger’s work. Ridley saw Giger’s stuff he was snowed. He said, ‘This is it!’” Scott called up Fox and explained that he would not do the film if Giger was not hired as the creature designer. His threat worked, and Giger was hired. Later, producer David Giler would tell Cinefantastique, “[Alien’s] a richly textured film, thanks to HR Giger’s work.” Dan O’Bannon agreed: “Only because Ridley was hired on was Giger hired. He took a liking to Giger’s work. Without Giger, I don’t think we would have had much of a movie.”

The direction for the design of Alien was also indicated by Ridley Scott to Giger, he told Giger the Alien should look like the Necronom V painting (thanks, BigDave) below:

Before Ridley  Scott was involved, the Alien looked like in Dan O'Bannon's

and Ron Cobb's visions:

It worths a mention that Ridley Scott chose Bolaji Badejo to be the man in the Alien suit (Strange Shapes):

"While Ridley was trying to find his Alien performer, Roger Dicken was skeptical that one could be found, telling Cinefex, “I went to about three meetings in London and watched these characters rolling around on the floor and quite frankly, I thought it was a bit Mickey Mouse. I mean, it was obvious to me that none of this was going to work, but I had to just sit around wasting time while everybody else figured it out. I sat through a few more meetings while they ran through football players and wrestlers and tall men. Then, for a while, they thought they’d use an ordinary-sized guy so there wouldn’t be any problems with stunts and all. At that point, I even offered to be the monster myself. I figured if I was going to make the suit, I might as well be in it.”

The saving grace was a trip to the pub. “We started with a stunt man who was quite thin,” said Scott, “but in the rubber suit he looked like the Michelin Man. So my casting director [Peter Archer] said, ‘I’ve seen a guy in a pub in Soho who is about seven feet tall, has a tiny head and a tiny skinny body.’ So he brought Bolaji Badejo to the office … I said, ‘Do you want to be in movies?’ and he said, ‘Sure’. And he became the Alien.”

“As soon as I walked in,” Bolaji told Cinefantastique, “Ridley Scott knew he’d found the right person.” Prior to filming, Badejo was placed on the Nostromo set with a mock-up Alien head and roamed the corridors on film, slithering, pausing, turning, kneeling, and prowling through the corridors to nail an appropriate system of movement for the beast.

“It’s very difficult for an actor to relate to what is, essentially, a beast. They know what it is, and they know there’s a man inside the suit, and they know the odds are they’ll never have to experience anything like it in their real lives … I think you’d probably die before the thing touched you anyway. I mean, you’d have a heart attack, right? You’d turn and see it and last about four seconds before you had a coronary, okay? So with Brett’s death, and subsequent run-ins with the Alien, it was always done with the ultimate feeling of a heart attack. The rush of a heart attack, even if the thing didn’t ever touch them.”
~ Ridley Scott, Fantastic Films, 1979.

Then we come to the design for the Space Jockey, after the two variants proposed by Ron Cobb 

and Moebius:

"None of these concepts were taken too seriously by Ridley Scott, who commissioned HR Giger to design the Space Jockey, using one of Giger’s Necronomicon paintings as a launching pad for the final creature" (Strange Shapes):

Here is the testimony of the creature master himself: “From the script I knew he was huge and had a hole in his chest, but that was all. Ridley suggested another one of my Necronom creatures as a guide. They don’t look much alike now, but it was a starting point; and the Space Jockey kind of grew up from there in bits and pieces. The creature we finally ended up building is biomechanical to the extent that he has physically grown into, or maybe even out of, his seat – he’s integrated totally into the function he performs.”
~ HR Giger, Cinefex, 1979.

And for the derelict, after the ideas of the derelict by Chris Foss:

and Moebius:

"Scott eventually turned to Giger, fresh from designing the Alien, for other interpretations of the Jockey ship.

“What we were looking for here was a totally alien-looking spacecraft,” Scott told Cinefex in 1979. “I didn’t think it would something with a lot of lights on it and stuff like that. I figured it would be like nothing anyone ever imagines; either that, or extraordinarily familiar and slightly archaic looking.”

“Once the Alien was under control,” said Giger, “Ridley asked me if I could design a spaceship not made by human beings. Well how do you do that? I thought maybe it might look organic -something that could grow even, like a plant- but I didn’t know exactly what it should look like. Then early one morning I couldn’t sleep, I got up and started painting and the derelict ship was born in a few hours. It ended up like an aerodynamic bone with little technical stuff all over it, but it wasn’t anything I had planned – it just sort of ran out of my mind and my airbrush.”

Like the other artists, Giger brought his own style to the concepts, and his proved to be the most provocative. His Jockey ship rests atop a landscape of twisted metal and bone. “I wanted it to look planted,” Giger told Famous Monsters, “perhaps in the process of maturing, a mixture of organic and mechanical stuff.”

“Giger’s first drawing was just a knockout,” said Scott. “I took one look at it and said, ‘That’s it.’ Other people couldn’t quite see it though, so I had to keep digging my heels and saying, ‘You wont get a better derelict – don’t screw about with it.’ You know, Giger is a special case, and when something’s that good, you have to recognise it and leave it alone.’

"However, despite Ridley loving the design, some members of the production crew took issue with the derelict’s odd shape. First to object was the film’s writer. “O’Bannon,” Giger wrote in his diary, “who has just flown in from the USA, doesn’t think it’s technical enough. A battle of pros and cons begins. I keep my silence; I know that Scott will win the argument.” Of course, as it turns out, he did."

(Strange Shapes)

Thank you for your patience to read such a lengthy post.

And thank you, Ridley Scott, you are my hero!