I don't think this should be as much "which sequels need to be over" as it should be "developers need to get some real development going before releasing a sequel".
I mean, Halos 1 to 3 are basically the same game with a couple extra guns and maps and a slightly different plot... Halo 2 and 3 might as well have been expansions that nobody would notice a difference, I mean, I've seen expansions with more actual content added...
But Halo isn't the only one, a crushing amount of developers these days seem to suffer from "EA/Nintendo Syndrome". It's not about releasing a good game, it's about releasing as many games as humanly possible, generally copies of each other, that LOOK good. But as yathzee put it "they always LOOK good"...
I have nothing against sequels, no matter what the number... I have something against franchise milking sequels, that really bring nothing to the table.
I mean, has there been 1 year since like...1998? 1995? Where EA didn't release yet ANOTHER FIFA/NHL/FHL/NBA..etc tittle or MORE? It's always the same fucking game with slightly better graphics and 1 different control.... No, not even... Just slightly better graphics most of the times... And people buy it, which is the root of this particular evil...
If a developer has REAL content to add, they should make a sequel... But only if there's actual new content. Otherwise, if they ignore this and make yet another copy with a new name, don't tell them "please do more!!" by buying the bloody thing.