Kind of feel this way about New Super Mario Bros. The first one, on DS, was a lot of fun. But Wii, NSMB2 and U didn't do much for me. I still finished them, because I think those are still very well made games and deserve to be given a chance. Still don't really enjoy any of them after the first though.
It's a small series, but Project Zero/Fatal Frame peaked at the second game for me. I rank it as one of my favourite survival horror games, 1 was decent and the sequels 3 and 5 (4 didn't get released in the west, but I hear it's bad) are ok, but they never felt as good.
It's pretty hard to think of more, I guess I agree with Soul Calibur, I loved the Dreamcast game, after that Soul Calibur 2 was a bit of a letdown, though Soul Calibur 3 did get me back into things, but since then I've barely touched the PSP game, 4, 5, don't even mention that free to play one and haven't even got 6 yet. Similar situation with Tekken, where it kind of peaked at Tekken 3 for me but I still enjoyed them as far as Tekken 5, before dropping out. But recently got back into it with 7 so maybe Soul Calibur 6 will win me over.
Command and Conquer peaked with Red Alert 2, even though I still played later games, I couldn't get into them nearly as much.