Well, everyone has already mentioned the ones I´d recommend but I wonder if you have ever played "Sam & Max Hit the Road".

Thanks for all the answers, guys! Yep, I had played a bit in the past...was amazing! (Also nice to see people from Uruguay :P) Had forgot to say, I've a savegame of Sanitarium completed up to chapter 7 somewhere, If memory serves.
That game is amazing indeed!

Okay, Gabriel Knight and The Longest Journey get mentioned a lot... in all honesty, I had forgot to add TLJ on the list! It looks amazing (and lenghty), but is it true that the sequel is a letdown?

Anyway, thanks for the answers, everyone!

Agreed, the setting and story were what made TLJ great.
Well, perhaps it's fixed now but on release the iMuse was screwed up and the opening credits missing.
http://nukezilla.com/2010/07/12/unconvinced-pete-imuse-and-monkey-island-2-a-follow-up/

Totally broken in the 360 version but still messed up in the PC version: http://youtu.be/JkMoHEFtnLQ?t=56s
It's mistimed here for example. Even worse is the fact that the problems in the modern version carry over to the 'original mode' because it's still the Special Edition's version of iMuse.
http://youtu.be/JkMoHEFtnLQ?t=2m27s

My brothers and I played Myst back when we were in grade school and we loved it. So it's not impossible for beginners to enjoy it. Practically speaking, however, it demands a pretty brutal level of abstract reasoning, and a degree of focus and patience that many people would find tedious. It's for a VERY SPECIFIC TYPE OF PERSON. The type of person who loves spending a lot of time puzzling over and exploring a world, and doesn't mind having to actively manufacture common ground between them and the game.

Gemini Rue is pretty awesome and has regular flashes of brilliance where the storytelling is concerned, although I'd dispute it being "almost flawless."

Did anyone mention Machinarium? Because Machinarium is amazing.

I heartily recommend the Gabriel Knight and Broken Sword series.

And anything touched by Wadjet Eye.