So my selection are the four villains whom I believe bring the most bad-ass and diverse flavour to the Squad.
1) Saruman - The Lord of The Rings
I see the White Wizard as my team's field leader. He has a commanding personality, a fearsome opponent in battle and can bring allies to the Suicide Squad through his White Council. So what if he died in the trilogy; his spirit may have left Middle-Earth but it could find its way to a resurrection so he can use his dark sorcery for a suicide mission.
2) Fish Mooney - DC Universe
Bringing girl power to the team, I see Fish challenging Saruman for superiority. She is a gangster boss who does not cower to any man. This is a woman who ripped out her own eye with a spoon just to gain control. She has her own gang as allies to the team and again might not really be dead, maybe she has swam to sure.
3) The Mountain - Game of Thrones
Every team needs muscle, muscle doesn't come more heavy-handed than this brick-(bleep)-house. I see Gregor Clegane in a King Shark (possibly Killer Croc) type role of being the one to be detonated but he shows loyalty to his superiors and so will obey his commander.
4) Storm Shadow - G.I. Joe
Presenting the team's experienced assassin. Every team needs a Deadshot-type character and anyone who has seen the movies know that this ninja fits the bill. He has connections to COBRA which could help the team with military weaponry. Storm Shadow also brings further diversity to the team (Saruman with the age, Fish with the gender and ethnicity, The Mountain with the size and now Storm Shadow with the ethnicity again). Finally I think maybe Storm and Fish can have a little bit of side-action á la Fish and Nikolai the Russian?
So these aren't in my 4 those are above but I wanted to add a couple of people for reasons I am going into now.
Demon Headmaster - Commander
While I love Amanda Waller I think she is too much like Fish, also with the Demon Headmaster's hypnotic powers; I feel he will be more persuasive in getting the team to comply.
Sweeney Todd - Backup Member
So if for some reason someone is detonated then if the mission requires four my reserve is Sweeney Todd. Pull him out of time and get him in that team, he is what the Suicide Squad was made for. He would most likely be the Joker in the pack.