After my first playthrough, I was disappointed in my Reuniclus, but then I checked the nature: it was Adamant. After that letdown, I tried one again and was blown away by the PPPPPOOOOOOOOOOWWWWWAAAAAAAAHHHHHH!!!! (Sorry about that).
As for actual disappointments, Samurott was definitely one worth mentioning. It just wasn't strong enough later in-game, but then again, there weren't many Water-types in this generation. Once I beat that run-through, I decided that Samurott was definitely the worst Water starter.
Druddigon and Liepard were awfully disappointing as well, looking cool, but in actuality being quite weak.