That's not going to be so easy, I'd wager. Not going to put them in any kind of order and will mention two games sometimes because I'm awesome like that and I like to list things. Also I will have forgotten some...
Anyway that post is far too long already...
[ul][li]Baldur's Gate II
Blew me away when it was released: atmosphere, characters and storyline is, beyond a doubt, great and it's still my personal reference when it comes to quests.[/li][/ul]
[ul][li]Civilization IV
Every single game is a new experience, every single war it's own story and every single interaction another event to be recalled for years to come. I don't know how many hours I've put into that game, but I'll be damned if it isn't one of my favourites.[/li][/ul]
[ul][li]Gothic II + Night Of The Raven
If Baldur's Gate II is my reference for questdesign, G2 is perhaps my reference for open world games. The whole game lives through it's handcrafted world that offers something new to discover at every step. It's difficulty doesn't scale and every time you beat up some monstrous troll through cunning tactics (i.e. outwitting the far too dumb AI) is a hugely rewarding experience.[/li][/ul]
[ul][li]TES IV: Oblivion & TES V: Skyrim
Oblivion is a strange game. It shouldn't really get me like it does; After all G2 is simply better in everything except perhaps some of the quests. Still I played it and played it and still play it. I blame mods. Skyrim is the same. Though, at least, the world is much more immersive than in TES:IV and it has much better gameplay mechanics. It's quests are definitely worse than Oblivion, however. But there will be mods. And those mods are cool. Oh and did I mention the mods? [/li][/ul]
[ul][li]Heroes of Might & Magic 5
Honestly if I called Civ IV addicting, I can call this an obsession. I have wasted so many hours in every single game from this franchise, that it's not funny anymore. The biggest drain being probably HoMM5 as it was (and still is) installed for many years on my laptop. The ingenious mix of fast tactic battles and RPG elements is a recipe for getting me into that whole "one more turn syndrome". Also worth of note: I have yet to completely beat a campaign in any one of those games. [/li][/ul]
[ul][li]Quake III Arena
The quintessential shooter. It's speed and simplicity is still unique among any kind of fps. Just shut your brain off and enjoy the most stressful and adrenaline-pumping multiplayer experience I have ever had. [/li][/ul]
[ul][li]Bioshock
Did show me that shooters can actually be smart and immerse one in a kind of atmosphere I had previously only encountered in RPGs. It's gameplay may be bland but it's one hell of an experience for me.[/li][/ul]
[ul][li]The Legend Of Zelda - A Link To The Past
For some reason I never mention the Zelda series when it comes to my favourites. And that's despite having played every single entry on a major console (i.e. no handhelds and no Phillips-CDI crap - also no Zelda on the Wii because I'm bored now) - I guess I'm simply far too saturated with the Zelda formula now. Still the one I played through the most has to be mentioned: Zelda 3. It's a game that improved upon it's predecessors in almost every way and was full of stuff to discover and collectibles to find. It's one of the best games on the SNES.[/li][/ul]
[ul][li]Super Metroid
Oh Super Metroid. One of the few games on the SNES I still play through often enough. It's a great game with myriads of things to discover and a haunting atmosphere that I don't think any game of the Prime franchise could emulate that well. It's just too good.[/li][/ul]
[ul][li]Megaman X
Back in the day I always traded in my used games to get something different when I was bored once again. It was, without a doubt, a giant waste of money. Still, one of the games I always returned to was Megaman X. It's a game that shows how a great action-platformer has to work. It improved upon the already awesome Megaman formula, looked great for its time and was addicting to boot. Also the soundtrack fucking rocks.[/li][/ul]
[ul][li]Master Of Orion II
Another far too addicting one. There is simply not a single 4X game up to this day that has it's appeal or insane functionality. A minimalistic but tactical combat system, many possibilities of specialization and an eventful journey like Civ 4 with every game - only with Death Stars. And I like Death Stars.... [/li][/ul]
[ul][li]Xenogears
A game that is criminally underrepresented for my tastes. It's basically Gnosticism - in space and in japanese. It features all those little annoying things I despise from JRPGs but the story is one of the most enticing I have ever played. Not for it's intricacy but for it's literal epicness.[/li][/ul]
[ul][li]Chrono Trigger
Played it once. Exactly once, years after the SNES went out of fashion. And despite that, I still knew that this was by far the best SNES game I had ever played. It's story was enticing (much more than FF6 mind you), the soundtrack is awesome and the battle system worked - especially since it didn't feature those annoying random encounters but had the decency to show you the enemies on screen. Oh and Yasunori Mitsuda's soundtrack is, simply put, amazing.[/li][/ul]
[ul][li]FTL & Minecraft
Two games again, since they are both indie titles which are a little bit too recent for me to be among my favourites yet sill managed to bind me like not much else: FTL & Minecraft. Where FTL is the incarnation of the Stockholm syndrome, Minecraft is a modern lego set that lets me dive into those cheerful past days of brick-induced joy. [/li][/ul]
[ul][li]Rollercoaster Tycoon & Transport Tycoon
Yay! More addicting strategy games! Seriously, those two did cost me another lot of hours over the years. I built whole theme parks based on ancient empires and companies that spanned whole archipelagos I built myself...ahhh nostalgia.... [/li][/ul]
[ul][li]Planescape Torment & NWN2: Mask Of The Betrayer
I'm actually a bit unsure if I should put them here, for the simple reason that I've played both only once. And yet...they are such great examples of how a great story can look like and what games can achieve in terms of philosophical intricacy that I simply have to put them on this list.[/li][/ul]