Deus Ex: Invisible War
Like cogadh, I really enjoyed this game: And I played original game before. I personally think IW is a good game on it's own and since I didn't have any expectations, when I first started it, there was no disappointment.
UFO: Afterlight
... I can't help myself, but this game and its succesors suits me better than original X-coms
Warhammer: Dawn of War
I didn't buy it because of my friend, who have very similar taste in real time strategies. Now I play Dark Crusade (expansion pack) on-line every now and then :-)

I have to second feelings on DX:IW. It's a decent (if slightly average) game on its own right.
But fails completely as a sequel.

Judge Dredd: Dredd Versus Death
I just love Dredd himself, and his position on a lot, as well as the thought and atmosphere behind the Megacities. The gameplay is sub-par, but fine and dandy, at least as Dredd I find a reason to arrest everyone and anyone.

I have to say Army of Two for the xbox 360.
A friend told me it was boring a lot of review sites suggest it is mediocre. But I really find it quite good and enjoyable.

How I forgot about this one is beyond me, though I suppose the first time I tried to play it I was unimpressed to say the least, I guess I was looking for Deus ex 2.
But after having retried playing it after some weeks I came to realise what A great stand-alone title it was, it's just unfortunate enough to have to follow in the footsteps of A true great.

Deus Ex: Invisible War is a good game that's also a bad sequel.

Now this is an interesting topic.
Let's see, for me probably Ultima 8. I actually liked it, even though it's gotten a lot of hate.
Not the same kind of game as U7, sure. But still good.

Kane and Lynch, though not so much "loved" as "appreciated". The gameplay was competent, but only competent, and some elements of the plot was fairly contrived. However, several aspects of the game really stuck with me:
-I found the voice-acting for Lynch on the whole to be very good (I felt he managed to imbue a lot of moments that could've been wangsty with a certain amount of "genuine" pathos.)
-The scene at the beginning of the construction site level was genuinely moving. Not that I was crying over my keyboard, but the game got to me a bit, which is unusual and praiseworthy
-Ditto for the final sequence, either way you play it. The way that a given outcome is forced (trying to avoid spoilers here) really frustrates me as a gamer, but I do appreciate what it does for the narrative and for your emotional investment in Lynch and his moral development.

I remember loving Hidden & Dangerous to death back when it was released, even though it got something like %50 in PCGamer.
Another one was Star Wars: Force Commander, nobody I know like this game, yet I had a blast with it all the way through.

Jurassic Park: Trespasser is supposedly one of the worst FPSs ever. I loved every second of it. Sure, it was buggy, but it was like a Half Life 2 + Far Cry... around 7 years earlier!

Following that logic, Fallout Tactics is a great game that's a terrible sequel.[

I just remember that... I liked Mortyr.

Call of Duty 4.
I really think it's broken in many places and has terrible racial stereotypes which make me cringe. But sometimes I pick it up and play it like 4 hours in a row, having really fun before I start hating it again.
Update: Just noticed this is a bit off topic... well whatever :)

Another +1 for Trespasser. Despite the interface issues and the relative simplicity of the physics puzzles, the game is fantastic. I can't think of any other game that matches the feel of genuine exploration that I got from Trespasser.

Alone in the Dark (The New one)
IGN gave it a 3/10 for whatever reason. I played it and enjoyed it.