You need chewie to get through the early stages so power him up, Jedi consular deffo, I still use him now.
Ig-86 is very useful as he knocks off loads of points and calls a bonus attack, still have him too.
You have to level up most of the "free" ones just to progress but those are the only ones I have kept in my elite squad. I still use Talia ect in dark side only battles.
Geonosian warrior is also worth it too, similar to ig-86.
Characters to try and get are:
Darth sideous - awesome leader and triple attack
Luminara unduli - awesome attack, leader and group healer #1 person to get.
GUI Jon gin - takes a few upgrades to be good but very good triple attack and calls bonus attack.
Count dooku - 100% counter attack, once levelled up he is great for dark side battles.
Captain phasma, great leader and gives counter attack to group.
Some characters work better with other leaders but upgrade ig-86, consular and genosian warrior, they are still used by most people and I'm at level 58.
Some guys have clearly invested money in it though, I've done it all free so far.