Yeah, @BMoney is giving you the clear answer, because that's what the devs want you to spend on and farm. HOWEVER, we've had a recent touch-up of Separatists, and it's hard to say now which of them would work in the Padme event (since we've never had the chance to use them as they are now in the Padme event). On the good side, your roster is so empty that basically all Separatists are on an equal playing field for you (it's going to be a slog no matter who you go with).
My purely subjective opinion, having had no opportunity to do the 7* Padme event with the newly reworked characters, is that you should farm zGG(L), zB2, zB1, zDooku, and then either try your G12 Ventress or if you're feeling masochistic, go for Nute. I can guarantee you good results with those Separatist droids specifically, and I can tell you that Magnaguard is not a game-changer. But Dooku plays well with Sith as well as Separatists, so he's worth getting (more so than, say, Poggle the Lesser). The Genosians with ships might be worth the trouble if you're committed to using their ships in your arena fleet, but if you're not, they're pretty lousy characters in and of themselves.
I've recently been having fun (e.g., TW/TB, not arena) using zDooku(L) with Nute and Talzin (I see you have her 7*, if not supergeared yet), because Dooku's lead causes people to inflict Tenacity Down and Healing Immunity on themselves when they drop below 100% health... think of all the horror with Talzin's plague on everything! Anyone who inflicts status effects, in fact, would work well under his lead (again, for fun. I don't think it's arena viable. But you might surprise someone in GA or TW/TB).
Sorry for going on and on, but I've been working on my Separatists and thinking about this stuff as well at the moment. Hope it gave you something to think about! =)
SamuraiUX
