I think I am going to kinda break the console-friendly flow of this entire thread, but... Here is my list consisting of a whooping 7 games.
1.
a) Little Big Adventure 1 / Relentless: Twinsen's Adventure
b) Little Big Adventure 2 / Twinsen's Odyssey
I will take either one of them over Diablo any day. And if you haven't played them, you're missing out on one of the best isometric "adventure RPG" (you level up with items, and you have no stats to speak of).
2. Freddy Pharkas: Frontier Pharmacist
Without a doubt the best game Al Lowe has made. I adore the Leisure Suit Larry games, but this one takes the cake. It's one of the few games that Sierra has done that contained some weird-ass time-based puzzles, and was also the only adventure game _I_ know of to be set in the old west. Seek this one, it's worth every minute.
3. Zak McKracken and the Alien Mindbenders
It saddens me to see that Maniac Mansion, Loom, and Sam'n'Max titles are always mentioned as something magical and amazing, uttered by the fans that remind me (mostly) of Tool fans... They are people who think they 'get' something others don't, because they've played Loom and others have not. But Zak McKracken gets mentioned almost never, unless you're in Europe... It is argued that the reason for it's European fandomship is the presence of NOSE AND MUSTACHE GLASSES in the game, which, as you already know, is the most favorite toy #1 in Europe [citation needed].
4. Darkwing Duck (NES)
The bosses were obscene in this game. They were hard, and the controls were not very fun. Despite that, the fact that every level began with the one and only protagonist falling onto the stage FROM THE SKIES while proudly proclaiming I AM DARKWING DUCK! (complete with the exclamation sign) made it a worthwhile endeavor. Having said that I love it, it is only the only game so far in the list that I have not managed to beat, because the last boss appears to have some sort of a mechanical masturbation device, which is as deadly as it is addictive.
5. Broken Sword: Shadow of the Templars
I feel sorry for every person who doesn't know what this game is. It's so good, I won't even write anything about it. Except that I love it with every molecule that comprises my body and possibly soul.
6. Psychonauts
I bought that game 7 times so far, and I still do not own it. After playing my friends' copy, I've been buying it for every friend that have not heard of it, sometimes compulsively. The genius of this game is unquestionable, and if you don't like it, I'd appreciate knowing that, so I can know how many of you have sold your motherloving soul.
7. Heart of Darkness
I liked Eric Chachi ever since he made Another World / Out of this World (depending on where in the world you live). When Flashback came out, I was hoping it was his, but I thought "damn, this is like Prince of Persia, but not as good as PoP2..." Well, Chachi went on to make a truly epic ass-kicking side-scroller known as Heart of Darkness. I never owned the game, and I do not know where you can find it now, but I've beaten it in California, while my parents took me to some guy's house when I was 14 or 15. Not only was the game GORGEOUS, but it kicked so much ass, that even Maddox would have to commend the lead protagonist of the game (an 8 year old) for being the ultimate master of bad-assery with a. lightning gun and b. green alien life force a.k.a. MAGIC.
That's it for me.