Looking back, and to narrow it down, I'd say the Toy Story characters simply because they all played the same, and figure-wise, their paint job was the worst. Even to this day, I still look in stores, and the paint jobs are just wibbley. I've never seen a "good" figure from that series I'd be 100% satisfied with.
Narrowing it down even more, Buzz Lightyear. He would have been one of my favorites, but again, all same moves. The fact that he lacked his laser and karate chop, and glide moves was just simply overlooking the most obvious features.
He's one I hope they "fix".