Cirque de Zale, for me. It was on a magazine cover DVD. Then after that I played the King's Quest Remakes (also on a magazine cover DVD), and the Maniac Mansion one (which I Googled, because I was in need of some LucasArts to balance out my Sierra). And then I also played Enclosure.
Before that, I had played some free SCUMM VM games (Beneath A Steel Sky, Flight Of The Amazon Queen, various demos), and a copy of the original, EGA Police Quest I found on a stiffy disk lying around the house. Oh, and Lure Of The Temptress, which I never played much, due to it being rubbish.
Around this time a year ago I was referred to 5 Days A Stranger by a friend's review, and that's what really got me into playing AGS games. 5DAS branched out into the rest of Chzo Mythos, and then into the rest of Yahtzee's games, and then into Ben Jordan, and then, and then, and then.
I now have well over a hundred adventure games on my computer. Yay!
At the time I played it, Cirque De Zale really amazed me, or rather, its credits amazed me. I was astounded that one person could be so multitalented as to make pretty much the entire game by herself.
I was a bit annoyed by the length of the game, though, having been spoilt by epics like Beneath A Steel Sky and Flight Of The Amazon Queen.
Overall, though, I think Cirque De Zale has made me try to hone as many skills as I can. I program, I design, I write, I make sound effects, I pixel graphics (not terribly well, but I'm getting there), and should really start trying to make music.
Cirque De Zale also showed me that adventure games don't have to be 15 hours long in order to be enjoyable. =D