Master of Orion 3

Bought on release for full price (around 50€) and was expecting improved MoO 2. Boy was I disappointed.

*goes cry in a corner*

This. First hint that something was amiss was that the max resolution is 800x600, coming from Commandos 2 with its max resolution of 1280x1024. Of course it makes sense when you see how much smaller the maps are :P
Though perhaps its not actually the worst sequel actually. That 'honor' might belong to Commandos Strike Force, but since I haven't played it yet I'm not sure.

Also a thread like this isn't complete without a mention of the Sacred 3 travesty.

All Deus Ex games but the first and Human Revolution
Fallout 3, even if i knew beth would have fucked up, as they usually do
Star Ruler 2, where everything unique was gutted out of it, turning it into a standard 4x game
Starpoint Gemini Warlords, pretty much no story or development at all, plus no new mechanics like custom ships, multi-platform turrets or advanced AI

I guess that as a sequel to the previous Commandos games it's kinda like what I said about World of Warcraft, though on it's own I actually thought it was a solid FPS. But maybe I was just never invested in the series enough to care about the "betrayal" - I always liked the various Commandos clones like Away Team and Robin Hood Legend of Sherwood more than Commandos series itself.

SimCity 2013

Hm, I think I've got nothing. Or do I?

The only sequel I remember feeling pretty pissed off about was Mass Effect 2, with the ridiculous story premise, the sudden change in tone, the silly over the top sexualization of some female characters and uniforms, and all those annoying huge pop up messages on the screen all the time. But I eventually got over it and learned to like the sequel for its stronger parts (which is definitely not the main plot, but the sidestories and companion missions were fun). Mass Effect 3 started (and ended) weak as well, but by the time I played it I was ready to look past most of its flaws and just concentrate on the good stuff again.

I disliked what I've seen of Escape from Monkey Island, but I guess after Curse of Monkey Island (which was still good but not that original anymore), I didn't expect that the series would return to former grandeur.

Neverwinter Nights 2 was a bit of a disappointment as well in that for every improvement over NWN it seemed to do at least one thing worse again. But it's still a nice enough game.

I found Risen 2 boring compared to Risen, but maybe I was also a bit fed up of playing the same old same old and I was already aware that it's not as highly rated, so it wasn't a big deal. I just dropped it and played something else instead.

I don't know, either I haven't played that many really, really bad sequels yet or I hardly ever feel as much negativity towards games and sequels as others do.

Well, I might be back when I've played Beyond Good & Evil 2. The concept doesn't sound too bad, but the trailer was awful.

Risen 2: Dark Waters

I stopped playing during the first level or so. I mean, the game completely disables movement while it makes you listen to unskippable tutorial messages. What were they thinking? The other TR games didn't even have tutorial messages and didn't need them either. Ok, maybe tutorials were all the rage at the time or something, but I think I've never seen any other action game that disables your character movement like that. "Press jump button to jump, but wait till I say so. For the time being, just freeze." :P

Hard to decide for one.

FF13 (and 13-2 and LR since the following games were even more disappointing than the already disappointing game which came before), same with DA2 and DAI.
DE:MD especially because of the Microtransactions (I never used them, I'm not THAT stupid but it and the Ending still made the game disspointing for me).
Star Ocean 5, I thought it can't get much worse than SO4...........
ME:A do I have to eyplain why? I don#t think so. Luckily I only borrowed it (uses the Origin Account of a friend) I learned my lesson after DA:I not to buy EA-games.

Both successors to Guardian Heroes (1996). The actual sequel "Advance Guardian Heroes" (2004) felt like a weak follow up. The supposed spiritual successor "Code of Princess" (2012) felt like it had almost nothing going for it other than its humor imo.

Normally I can give a game a break up to a certain point, but really, with these two I have a hard time forgiving them. It's not even about them failing to live up to Guardian Heroes, I just found them hardly enjoyable even on their own merits. Given the amount of time between each game I hoped they would've kept pace with or surpassed Guardian Heroes, but the few things they brought to the table just never made them feel like a better experience.

Fallout 3

Invisible War, while not really bad, is pretty disappointing compared to the original. And I found it very difficult to follow the plot.

Space Quest 6 was also awful, right off the bat it just sucks to see Roger no longer be a ship captain, and also dropping the Beatrice romance sucked after that having spent two games developing.

That's easy, Command & Conquer 4. Never before has a video game sequel made me feel actual anger. I broke the disc in frustration and every time it comes up I start to feel pissed off and agitated. Fuck that game.

god... easy, Dead rising 2. game was laggy, had a graphical downgrade for some reason but taking a piss was the best save feature ever made! lol

Came to say the exact same thing. And I really, really like the new direction they took with C&C3 & Kane's Wrath, too. What a waste, and a franchise killer.

Raiden 3 is a mess compared to the previous games. Thankfully we have the awesome Raiden 4.

I guess 1st place has to go to Diablo 3. The gap of quality and value between 2 and 3 is the biggest of any sequel I can think of.