Seismitoad was a bit of disappointment for me. Palpitoad/Seismitoad was one of those really good midgame Pokemon that just gets totally outclassed by the late game. I used Seismitoad in my first playthrough, and in that playthrough I used Samurott more, even though it had a really crappy nature and IVs.
I guess since I brought him up, Samurott is mildly disappointing. He's good, but he's not great, which is the drawback of him being so damn balanced. He never really seems to excel.
In the one instance I used him, Golurk was underwhelming. I spent a TON of time leveling up Golett on Victory Road to get a Golurk before the Elite Four, and ended up not using him at all because he just didn't seem to be better than other, more economical options.