"Best sequel of all times" is a tall order, but if I think about it this way, that the sequel was not just an improved version of the first game but something that was exponentially more fun because I wasn't all that fond of the predecessor, then definitely Baldur's Gate 2, with runner up Dungeon Siege 2. Oh, and Saints Row The Third, obviously.

The "quiz" says:

Your Winner
System Shock 2

Runners Up

2. The Elder Scrolls II: Daggerfall

3. Warcraft 2

4. Max Payne 2

5. Assassin's Creed II

But the quiz had many shitty options and not a whole bunch of great ones. But at least it had two of my favorites and sequels that are predecessors to my favorites from the series (Worms 2, Civ2 and Daggerfall).

My actual list would be:
1. Worms World Party
2. Sid Meier's Civilization 5
3. Elder Scrolls 3: Morrowind
4. System Shock 2
5. Warcraft 2

My top list is this :

1 : Last Ninja 2 - C64
2 : Diablo 2
3 : System Shock 2
4 : Silent Hill 2
5 : Borderlands 2

Others that I really enjoy are :

Hunchback 2 - C64
Rick Dangerous 2 - Atari ST & Amiga
Monkey Island 2 - Amiga & PC
Soul Reaver & LoK sequels
Mass Effect 2 & 3
Thief 2 & Deadly Shadows
Hitman Absolution
Rainbow Six Vegas 2
GTA Vice City

Morrowind is my favorite game of all time so perhaps that's the easy choice.

However if we think of sequels as a return to a series or place we loved with new added features and a more modern look then I'm not sure Morrowind as a sequel to Daggerfall really works as well.

In that case I would give it to Dees Ex: Human Revolution or Fallout: New Vegas, both of which are the perfect example of how to update a classic.

Thief 2

Wow... tough question but in no particular order...

Dark Souls (not sure if that counts...)
Red Dead Redemption (Not sure if that counts)
Assassin's Creed 2
Fallout 2
Torchlight 2
SimCity 2000

That's all I've got right now...

If I may I'd like to jump in again and mention Dark Reign 2 not because it's that great but because it was so much more improved over the first one. My hats off to the programmers acknowledging that things had to be fixed and addressing those things.

That one might actually get a high spot on my "worst sequels" list. :P

According to that thing:
System Shock 2
Civ 2
Portal 2
Fallout 2
Dark souls

Coop campaign was a blast with my brother. Because it was heavily patched over the time. It's the last game I played more than 300 hours I think.
But I completely understand your point of view :)

I also played it with my brother but neither of us could really get into it. None of us liked the actual multiplayer mode of the game either. Well, at least the last mission in the campaign actually gave me goosebumps. I'm wondering whether the addons are better.

My biggest hope is that DoW 3 will be more similar to the original game again but looking at all the stuff Relic has been making after the original Company Heroes I don't have high hopes for either series anymore.

My list was:
Chrono Cross
Silent Hill 2
Resident Evil 2
Super Mario World 2: Yoshi's Island
Banjo-Tooie

My real list would be pretty close:
Chrono Cross
Silent Hill 2
Final Fantasy VI
Super Mario Bros. 3
Resident Evil (GameCube Remake)

In coop, my take is to play on the hardest (hard the first time and hardest after), because I think that trying to survive dire situations is the most fun thing in the game (and playing coop simplifies microgestion a lot).
The add-ons campaigns provides :
- more story with less content
- but way more replayability I think.

Anyway the sure thing is that the game is far more enjoyable with a friend.

Sidenote, it's one of the only games I know that have a native 3 players coop mode (The Last Stand) ; which is really awesome if you have 2 friends who already are veterans of DoW.

Call me crazy but for me it's...

Jet Set Willy 2

The first game was absolutely amazing but the sequel? More than twice as many rooms, secrets, space, sewers... it was the first time when a sequel blew my mind so whenever somebody asks about a favorite sequel my answer is Jet Set Willy 2 Just check out this amazing map