Super Mario Galaxy. Especially considering that they padded out the sequel with obnoxious green stars that were mandatory to collect before you could get the final level; stop saying they were an optional challenge because they weren't.
Sonic the Hedgehog 3/Sonic and Knuckles. Basically they were still making Sonic games left and right already. There was no reason to call one of the newer 2D games Sonic 4, especially when what they did call Sonic 4 played nothing like Sonic 1-3 [http://www.youtube.com/watch?v=f6-1cGzPn3E] and thus wasn't a sequel in anything but name. The game that is being called Sonic 4 should still exist. It just shouldn't be called Sonic 4.
You don't see them making "Sonic Adventure 3" do you? And if they did, same thing: What would be the point? They've still been making Sonic games long sense Sonic Adventure 2 came out with different names and plots; there would be no logical reason to just up and name the next one "Sonic Adventure 3". Just like naming a game "Sonic 4" was not needed.
Mega Man X5. It was supposed to end there, but some guys made X6 and it sucked. So they they made X7 and that sucked (slightly less, but it still sucked). Then they made X8 which wasn't nearly as bad as the last two, but still bad when compared to the much earlier X games. And Command Mission, although not outright terrible like X6 and X7, was still completely unnecessary.
Spolsion Man. Wasn't it bad enough on its own without having to copy off Ms. PacMan for a sequel concept? And this was at the same time that Twisted Pixel was whining at Capcom for making their own version of Splosion Man, saying that Twisted Pixel works hard to make original games and they're upset that someone ripped them off. Yeah, I'd be upset too, but you don't look so original when you're making a sequel and you're copying heavily off of another game to do it. And again, I submit that the first Splosion Man wasn't worth a sequel anyway; it ran out of ideas by the time you got to the first boss, and then they still had what looked like 20-30 more levels for you to play. Screw that, the first few weren't even that good. Yes Twisted Pixel we get it, you substituted fulfilling platforming for a character that just explodes and makes goofy noises. Now go back to making awesome stuff like The Maw.
That's all I've got for now, I guess. Everything else I come up with is either just a franchise and not really a sequel, or it's just something where I hated the sequel, and a bad sequel isn't the same as not needing one.
