From 1.0 I definitely agree the Toy Story ones are the most disappointing specially with so much potential for them.
As for 2.0, I have to say Baymax so far is my most disappointing because I really find useless his special power. There was a bit of missed potential there.
Marvel wise, My most annoying one is Gamora because I don't like they chose Ventress' actress for her voice, as she exactly sounds like Ventress on this. I feel Ventress just took over Gamora's body or something lol. Groot is also a little too slow. And well Drax I only have him to have all the guardians.