I'd say Ralph is my least favorite. I hate his attacks. The cherry bomb is fine in concept, but it's harder to aim than guns are. It's wonky for what a ranged attack should be and slow.
And his basic attack is terribad. And if you play his mini-game in 1.0 [or whatever they were called] you know how dangerous that thing can be on small paths/areas. You move pretty far and it does little damage. It's just not good. Plus the sound effects of that attack drive me insane.
At least with Penelope her dash is useable, and the effects are really cool.
edit; This is more general disappointment related to characters but... it was really disappointing to not have a single female character in the Spider-Man set.
Not even a cross over female.
And people are making a good case for Toy Story characters being the most disappointing, but I find their dashes at least work to some degree. So I'm sticking with Ralph for my least favorite. Toy Story characters are super disappointing though.