(Note: This will not include Legendary or Raid Characters)
Cantina Battles: There are lots of good options here such as Lando, Hera, Ezra (Super OP), Old Ben, Biggs, Wedge (Best Rebel Combo other then Chaze), and so on. I recommend that you farm Hera, Ezra, Wiggs, Lando, CHS, Old Ben, and SRP from this area of the game.
Cantina Store: Pao, Bistan, Chopper. The only viable rebel here is Chopper and Bistan has some viability but is not good enough and Pao sucks and is part of the worst characters guild.
Arena: Lots of good choices such as Princess Leia, Kanan Jarrus, Stormtrooper Han, Cassian Andor, and Admiral Ackbar. Ackbar is the best rebel leader other than CLS because his leadership allows rebel allies to assist off of passive abilities. Leia works with an ackbar lead especially in the Sith Raid in Phase 2 when you have no other rebels in the team (Team is Nute/Ackbar, Leia, Kenobi, Sun Fac/Sabine/ST Han, Zarriss/Hoda/Wampa).
Galactic War: Zeb, Bodhi Rook, K-2SO. Only farm Zeb and K-2SO. Bodhi Rook is by far the President of the Worst Toons Club and this is not that good of an area to farm rebels other than Zeb.
Guild Store: Jyn Erso, Sabine Wren. Jyn Erso is another great F2P rebel leader that is not a legendary (You don't need CLS to beat this). Sabine is very deadly with her armor shred and is pretty good with Phoenix Squadron.
That Wraps up this guide. I hope you find this helpful and the force will be with you always.