Best characters to use in any team

You can add Rex and Nihilus to Goodgils list above depending on lead/non-lead needs and your level/age of arena shard.

Rex leader, barris unique zeta, GK crit immunity are some of the first that come to mind

Shoretrooper, phasma, papa palpi, bb8, many more

The usual term for these kind of toons on the forums is "plug and play". The idea is that you don't have to build a specific team that works with them, you can just put them somewhere and they'll do their work. Some lineups work better than others, but they work almost anywhere. The most popular plug and play toons right now are: CLS, Raid Han, Thrawn, R2, GK, Barris, Hermit Yoda, and Wampa - they can all work in any team.

I would also add Nihilus to the plug and play list. And Boba Fett and Kylo Ren.

Kylo Ren, CLS, GK, R2, JTR, Wampa, Probe Droid, Nihilus, Raid Han, Zeta Barris, Thrawn, Boba Fett, Old Ben, Ahsoka Fulcrum, Dooku, Royal Guard, Rex, and B2 battle droid. I feel like that's a pretty general list, unless I missed a big one, please add on, but for the most part I think that's about it that don't need their faction with them.

Thrawn