Old Ben - as your tank, i chose old ben over st han for end game viability. It may take you a long time to get GK and old ben can take you farther since his dodge chance is quite high and you have a slightly better chance of having taunt dispelled. However, st han giving turn meter is amazing early in the game, and you need scoundrels for credit heist.
Leia - she is the best overall end game character on that list and can be a good replacement for thrawn or han solo on a meta team if you dont have them yet. However, without r2 cloaking right before her turn, she wont get offense up amd her damage will be low.
Lando/Akbar - i cant really choose between them, but early on, Lando will be your star. He is also a necessary scoundrel. But you cant use his aoe against phoenix, dooku or rex teams. While akbar can be used with hermit yoda and cls, only a few people can swing an akbar lead in mid game, but not without atf and r2 and some more dps.