Darth Sidious features heavily in arena, i've unlocked and levelled him but i'm actually levelling a Droid team myself instead.
I have HK-47, next IG 88 and 86 then Poggle the lesser.
If you want better tank/ability block, go for HK first, IG 88 is squishy but has some powerful attacks, especially with other debuffs.
Daka is great for PvE levels, rarely seem him/her in Arena though so up to you.
Sidous' main benefit is his natural dodge against Jedi and heal blocking regular attack, his leader ability to up crits is great as well.