Most Pokemon I've used in DPPt were pretty good. I don't think I can single out one Pokemon which was in any way bad, apart from maybe Tropius in Platinum, but I kinda knew it was going to suck so I can't really call that a disappointment xD
Hmm, actually now that I think about it, I remember that Rampardos was pretty bad... very slow, and it didn't learn many useful moves through leveling up (and I was too greedy to use any of the valuable TMs).
In HGSS, Elekid (the one from Pokewalker) was rather disappointing. Stupidly frail even with the level advantage. I became bored with him somewhere around the 3rd badge and boxed him away.