Best separatist droid for non separatist droid squad

Well, if you want subs, i would recommend taking in magna

Depending on his gear, he can tank a hit from tee o in the challenge tiers. mod for protection
Droideka is great for single target damage, and very fun in the mythic tiers and below(mine is g11, no point doing anything higher yet). Getting a daze on droideka is great, as if you get damage immunity up, you can tank hits on it, and get 17+ stacks of charge. One unload obliterates paploo.

B2 is great, but he doesn’t have a taunt and his damage isn’t the best. If you pair him with droideka though( if you have the droideka zeta), b2 keeps taking turns, allowing droideka a chance to constantly assist

L3 or Jawa Engineer. You don't even need a lead. BB8 and T3 droid synergy with L3 might as well be good enough that it is considered a Unity lead instead of a single lead. L3 T3 BB8 R2 C3PO = crazy synergy. Maybe drop r2 for 88 with zeta. If you cant do r2 and C3 then z88 and zIPD have nice target lock synergy.
88 T3 L3 BB8 and IPD would be really fun IMO but they target Lock would suck against dispel teams.

So I'm biased but I'm a huge fan of Droideka. One of the rare toons who's mechanics aren't based around speed and I've seen him one-shot R7 Malak. Amazingly fun toon imo.