Favorites in no particular order:
Vivi, Auron, Squall, Snow, Zidane
The ensembles in 8,9, and 10 were easily the best, with the exceptions of Zell, Quina, and Wakka...
Least favorite, in a very specific order:
Aerith, Vaan, Hope, Wakka, Yuffie (in 7.. they improved her in everything else...