acidic blood

Maybe H2SO4 !..Acid Sulfur

The sulfuric acid would have to be pretty concentrated for it to burn through metal. I know some concentrated hydrochloric or nitric acid would really burn through that sucker well. It isn't so much the one paticular acid that is important, it is how concentrated it is.

If I remember well... hmm... Substances are divided in Basic and Acid... Basic substances can be very aggressive too it depends with what they come in contact...?!

Yes, acids and bases can be very aggressive, depending on the substances they come in contact with. I am not a chemist (sadly), but I have worked for a few years in a company that dealt with safety on chemical plants , and I've come across many stories of accidents with several chemicals (be them ionic or covalent) though the one that makes people cringe most (from my experience) is HF (fluorine can be very dangerous). I know that there are substances more corrosive than HF, but I don't really know them or to what extent they can damage the media they come in contact with.

I don't, no. But remember, the movie is/was "Alien", so I look at everything about the movie as being alien. Certainly there would be/could be substances that we're not familiar with. A quote from Aliens: "A creature that gestates inside a living human host - these are your words - and has concentrated acid for blood" So even in the movie, it was understood that the acid was strong.

Fluoroantimonic acid. Strongest of the "superacids". I know that in the Aliens Technical Guide, it has some (probably non-canon) discussions between W-Y researchers discussing hydrofluoric acid as being the base for the creature's blood, but just straight HFl is actually a relatively weak acid, despite its high reactivity.