I am rather surprised at all the hatred going towards Mass Effect 2. You guys all seem to be missing the point: This thread is about BAD sequels, as in, games that are properly bad. Mass Effect 2 certainly wasn't this, it's just that you found it disappointing due to a genre shift and gameplay changes. What emerged from Bioware's development offices was something different to Mass Effect, but something that still succeeded in what it was trying to do and be.
There are things I prefer Mass Effect 1 for. I love the Mako to death and would be perfectly happy with a game that consisted entirely of driving round on one massive planet (chock full of steep mountains to climb up, of course) in it. It was ridiculous, completely insane fun, and Bioware abruptly went and replaced it with... a somewhat-entertaining-for-a-little-while minigame that took up about half your time playing the game. I don't hate planet scanning, but I much, MUCH prefer the good old Mako.
The original game was also much more of an RPG in general, what with all the stats to upgrade and visibly getting experience points and loot from enemies you killed. I liked choosing between the many different weapon and armour varieties, and upgrading my gun with certain ammo types.
But in the end, Mass Effect 2 isn't an RPG with 3rd-person shooter elements, like Mass Effect. It's a 3rd-person shooter with RPG elements. This turns some of you off, and that's fine, but the fact is... it does TPS very well. The combat is better than the original game - it probably benefits from a somewhat smarter AI, quickly-recharging powers, a tweaked cover system (just pressing A to go into cover rather than having gravitationally attractive walls makes a hell of a lot more difference than it sounds) and better weapons... or should I say, better-balanced weapons. The story, I agree, is not as good as in Mass Effect 1, but it's still above par and draws you in. The character of Shepard feels a lot more realistic than in Mass Effect 1, no matter whether you're paragon or renegade, and the many other characters are also very good - I believe many of them are better than the original squadmates. I found all this sufficiently attractive to sell Mass Effect 2 for me, as, it seems, did many of the critics.
If Mass Effect 2 isn't your kind of game, that's fine. But claiming it belongs on this list just because it's a different kind of game to Mass Effect is ridiculous. It's like saying Halo 3 is a terrible game just because it's an FPS and you don't like FPS games.