Worst: Locke, his side-quest is one of the worst in the game thanks to the magic of selective amnesia, and even with his side quest/backstory he's still a extremely one-dimensional character. Whats even worse is the number of people who ACTUALLY THINK HE'S THE MAIN CHARACTER of FFVI, despite having little to no influence in the main plot other than being a part of the returners.
I guess some people consider Locke as the main character of FFVI just because they refuse to accept female as the main character. Most of FF protagonists have boring DIDs as their love interest : Rosa, Rinoa, Aerith, Garnet, Yuna. Though Celes had some DID moments as well, but really she was the only love interest who didn't suck.
Best - Zidane Tribal. He's brave and at the time, an interesting breath of fresh air. He has great back story, a love tale that felt believable instead of being ruined by an abhorrent kissing scene. His personality is almost always sunny despite his small moment of weakness. Though I hated his voice actor choices (English and Japanese) in dissidia and how his personality was dumb down to a shota kid with a 12 year old mindset , his image isn't slandered to me.
Worst - Yuna in X-2. I don't even feel like going into details on this one. I felt she would have made a better protagonist than Tidus in FFX, but when X-2 came around and she was actually filled the role, I instantly changed my mind (OK not really) ---
About Zidane in dissidia, blame Nomura/Toriyama for that I absolutly HATED for what they did to FFIX cast in dissidia... First Vivi in KH2 and now Zidane and Kuja in Dissidia(s) *facepalms*
If they changed Zidane's personality from his dissidia one to FFIX one I would have liked it more. And both voices (english mostly) would have been much more "alive". (now I don't make much sense here am I? :<)
I do blame Nomura and Toriyama for that part. Kuja, too, was just so terrible in that game >_< Though I think Akira did a great job of Kuja's Japanese voice. Wish I could say the same for the English one. As for Zidane. In Dissidia I felt he was turned into some little kid. He was pretty mature in IX and it showed through his personality. As much as I like Romi Paku (Japanese voice actress) I just didn't like it for Zidane. I also don't recall him being so short. His game style had a chibi look but in his world he should be considered standard size for a 16 year old teenager. I mean if he's taller than Garnet, then she would be a dwarf in this game. Instead of making him maybe a little shorter than Tidus and Vaan, they made him nearly 1/2 of Squall.
I would have liked Reuben Langdon, Jim Singer or Mark Gatha to voice him because Bryce Papenbrook sounds weird here. As for Japanese I would have liked Hiro Shimono.