Lets stick to positive comments; if someone mentions something you don't like, feel free to criticize the game elsewhere, but I'm hoping this thread is just devoted to glowing praise of awesome games and why you love them (not to say you can't cheer on other people's choices though). I'll start out with my favorites:
Resident Evil 4 (PS2 version)
There are some games that from the moment I pick them up I have to play them from start to finish. It doesn't happen too often anymore, but Resident Evil 4 was quite possibly the most compelling of them all for me (especially outside of the RPG genre).
I loved the feeling of being alone, surrounded by insurmountable odds, fighting all sorts of otherworldly, bizarre creatures. The way the monks chanted freakishly, the plagas that popped out of villagers, large numbers of enemies closing in from all sides; everything worked together perfectly with the slow but steady control style and emphasis on trick-shooting, accuracy, and ammo conversation to create a masterful survival game.
The horror part was debatable, but playing alone late at night certainly gave it that extra edge. I remember one night in particular it was 3 a.m. and I was running through the garden maze with the wolf/dog/plagas things, and there was an earthquake. It wasn't very strong, but it was enough to scare the crap out of me and make me think (in my half-awake, late-night state of paranoia) something was behind me moving my chair.
Decent storyline, fun upgrading system, awesome feel; and I especially loved the bonus content. Mercenaries mode and playing through on professional was incredible.
Final Fantasy X
This was the very first video game I have ever played that employed property voice acting along with strong narrative to create an amazing experience. I loved the Sphere Grid and damn near everything about the game up to the very end. It was really the first game I got on my PS2 that blew me away, and to this day I love to replay it from time to time just to re-experience the story. Itching to play it again now in fact, I just wish it would run on any of my current systems since I no longer have a PS2.
Final Fantasy Tactics
I'm not sure if I've spent more time playing any other game. Leveling up jobs, finishing sidequests, duplicating Excalibur... Ahh, those were the days. The remake is good too, I just wish it supported infrastructure networking.
I love tower defense games, and this one struck me perfectly. It's a great balance between predictability, style, challenge, and simplicity.
Francis York Morgan is the best character ever. That is all.