In the order I encountered them -
Purrloin - I only mention it because Liepard is so blatantly fragile. It didn't really "disappoint" me, since it's on the second route and I expected it to not be a late game pokemon. That said though, it almost works, since it's got a great movepool, and I suspect that if I really invested some time into it, I could build a nice one. Maybe.
Blitzle - I really had hopes for Blitzle and Zebstrika, in spite of the fact that it's such an early, and common, encounter. But it just never quite manages to pull its weight, though admittedly a good part of that isn't its fault - it would be a lot more useful if there was a better physical Electric move available. Spark just isn't powerful enough to serve through the game and Wild Charge's recoil is a bit too much for a pokemon as fragile as Zebstrika. There are better special Electric moves, but Zebstrika's only so-so on the special side. So it's left with lots of potential, but nothing really to do with it.
Woobat - it was obvious from the first moment I saw one that it was supposed to be this generation's Zubat, so I expected at least an evolution comparable to Golbat, and had a vague hope of ending up with something as good as Crobat. Instead, I got Swoobat.
That's honestly all I can think of. One of the things that impressed me the most about this gen was all of the pokemon that turned out to be better than I expected - Stoutland, Musharna, Excadrill, Lilligant, Leavanny, Conkeldurr, Sigilyph, Scrafty, Reuniclus, Gothitelle, Sawsbuck, Beartic, Mandibuzz, Braviary, etc., etc...