Opening up with a strong one here all movie fans will love, Evil Ash!
The leader of the rag tag team of rebels out to save the world, Evil Ash doesn't really need a team to lead because he can control the Deadites, but seriously... that has to get old sometimes, just walking bones that even a child could knock down doesn't sound too intimidating. Evil Ash would be the head honcho, leader of My Suicide Squad. Also if this were a comic book, a lot of great jokes and babbling could be done between Ash and Evil Ash.
Secondly, you'd need a completely deranged, man eating, psychopath, and who better than Hannibal Lecter? He would serve as... well, not really a title he would just cut you down and blab about some philosophical nonsense as he does it.
Because Boba Fett is too main stream and every group needs a strong bounty hunter, "will do anything for money" type guy, Bossk is the right fit for the position. Holding his own, and even winning against the fearless Bounty Hunter Boba Fett, Bossk has all the tools for the job and then some. He'd serve as the teams Heavy Hitter and nothing less. Step aside Killer Croc, or be blasted into smithereens.
Serving as the evil bad ass, Bellatrix LeStrange, she controls all sorts of evil spells and incantations to send even the most evil, murderous man, crying back to his mommy. Bellatrix serves as a great attacker and back up if need be. She'd just as soon laugh for no apparent reason than kill you on the spot.
And that wraps up my top four picks for my very own Suicide Sqaud! Hope you liked them!