Does it have to be a top ten?
I wrote out this big huge list of games that very clearly goes over ten!
But I suppose if I really had to boil it down to just ten games...
Yoshi's Island
The high water mark for all 2D platformers, especially Mario games. Incredible art style, a strong sense of character, interesting premise, and every level is unique. And for a franchise typically known for boring, repetitive boss battles, Yoshi's Island's bosses are still some of the best Mario bosses ever.
Donkey Kong Jungle Beat
The Gamecube version of this does wonders for stress relief and I have never once come away from it feeling anything but pure joy. Something about the combination of music and action and Donkey Kong's own over-the-top strength is incredible, as is his willingness to apparently wake up one day and say "I'm going to beat everybody up and become king of the jungle for no other reason than I can." I have yet to play the Wii version, but from what I've seen it looks like it is missing the point, a little bit.
Chrono Trigger
The best RPG of all time, as far as I'm concerned. Strong characters, an interesting world, a great battle system, and incentive for pairing up certain characters beyond "I like the vampire dude better than the ninja". Also one of Yasunori Mitsuda's best soundtracks.
Half-Life
The grandfather of the modern shooter, Half-Life is just scripted enough that you don't see the rails you're obviously being funneled down. Not the deepest or most character-driven story in the world, but that's fine by me - I prefer Half-Life's no-name scientists and generic security guards over Half-Life 2's name characters that must remain immortal or else you are sent to a game-over screen. What story telling IS present is incredibly subtle in a way that only Valve does best - what most videogames treat as "set dressing", Valve uses as plot devices. There's a fairly interesting narrative at the heart of Half-Life - but you have to be paying attention to know that it's there.
Castlevania: Symphony of the Night
The game that single-handedly changed my entire perspective on the Castlevania franchise from "I don't get what's so great about this, and it's too hard" to "a die-hard fan. Like Super Metroid with RPG-style loot drops, there's so much cool stuff in Symphony of the Night that I'm still learning new things about it even now, more than a decade after release.
Sonic the Hedgehog 3 & Knuckles
Me and my friends gave this game a whole entire summer trying to see all of the endings in the hopes that some wacky combination of Chaos Emeralds + Characters would reveal something as cool as the Doomsday Zone. This is the sweet spot of the Sonic franchise: not too many playable characters, not too few, and gigantic levels that utilize everybody's special abilities in interesting ways. I've actually been getting the itch to replay it again recently (though the same could be said of more than half the games I'm listing right now).
Grand Theft Auto: San Andreas
GTA4 might've been the first GTA game I beat, but it was San Andreas that really captured my heart. Something about the era - San Andreas doesn't really work as a narrative, because it takes our gang-banging hero CJ from life in the ghetto dealing with corrupt cops all the way up to taking part in government conspiracies and flying a jetpack. But as a snapshot of everything I remember from the 90's while growing up, San Andreas really touched a place inside of me, and also really fed my "What's over there?" syndrome - when a new island was unlocked, I would forget about the story and spend 3-5 hours just exploring the world. Some say that San Andreas was too big and too empty for them, but I absolutely loved being able to get out of the city and drive around in the woods, even if there was largely nothing out there except for trees. That was enough for me.
Parappa the Rapper
Parappa the Rapper is a game that is maybe 30 minutes long. But that 20 minutes is perhaps one of the greatest experiences on the Playstation - an interactive music video in the most literal sense, Parappa the Rapper exudes that weird "Japanese style" that everybody found so endearing in the mid-to-late 90's, while still retaining a heart (which is something that the sequels forgot to do). It wasn't until
fairly recently that I came to terms with my absolute adoration of Parappa the Rapper, but I couldn't be happier now that I've found it.
Portal 2
Not the most replayable game in the world if you ask me, but it also stands as basically the only game Valve's done to-date with a concrete ending with legitimate closure for its characters. And it's a hell of a sendoff in that regard - one that I am not afraid to admit made me mist up a little bit. Its puzzles might not be as devious as the first Portal's, but it's still a unique, interesting, and fun game. Almost perfectly executed in every way, as far as I'm concerned.
Left 4 Dead 2
I feel like this needs to go on here somewhere if only to justify the nearly
350 hours I've put in to the game these last two years (though if you look on my main Steam profile, it actually says nearly 600 hours, so who knows which figure is correct). I think a lot of that is probably just the group I play with - but it's also just a great idea. Left 4 Dead 2 is always familiar, but never too familiar. You're never quite sure what combination of what you're going to get where, and it creates just enough variation that the game has near infinite replay value for us, I think. I just wish there was more - I want to know how these stories end, and outside of Portal 2, Valve seems to favor leaving their fans hanging more than they do giving closure.