I'd say that every character is an antagonist in this film.
I was thinking the same thing as I typed, but you know what I mean. I will be disappointed if it is the Joker. He's not what I think of as a Suicide Squad baddie.
If you discount the villains then it's probably Batman, but I hear he only has a cameo in SS (hope not ). It didn't seam like Joker was part of the team from the trailer, so that's always possible. They usually take a lot out of context in trailers though, so anything is possible.
To my knowledge, the only actors roles that haven't been revealed are Scott Eastwood, Ike Barinholtz, David Harbour & Alex Meraz. It wouldn't surprise me to see the new Robin show up in this film though.