It's going to really depend on how "serious" they want the movie to be.
Trelane I'd put at 50/50, he makes for quite a good villain until you find out he's basically a child with godlike powers masquerading as an adult.
Gary Mitchell, if the Star Trek movie is going for a more serious bent I'd go with Mitchell over Trelane. I mean killing your best friend to save your ship and the universe can make for a strong story. Granted, it all depends on how well they write the Kirk/Mitchell relationship and if the two actors can come across as friends.
Talos IV, the upside is that it would make for a really interesting story trying to escape from Psychic beings who want you in their zoo. The only down side is that it all takes place on one planet so there is much for the ship to do.
The Horta, basically would take the idea of the "Alien" movie and turn it on its head. I could see it play out as a Sci-fi Alien suspense movie and then you find out the Horta is only killing out of self-defense. It would take a lot of guts to adapt the original story as is, instead of turning the Horta into the next Xenomorph.
Harry Mudd isn't really a villain, he's basically a two-bit conman who's schemes sweep up the Enterprise. He's not really out to screw over Kirk and company, they're just along for the ride and have to figure a way out of his mess. He's more of a McGuffin than a villain. If it's Harry then on of two things will happen, the movie will be much lighter in tone or they will totally change the character to be more like Doctor Smith from the Lost in Space movie.
If they don't do any radical changes to the characters or story I'd say Mitchell or the Talosians if they were doing a "straight" sci-fi movie. If they were going for slightly siller I'd say Mudd and whatever mess he creates or possibly Trelane.
If they wanted to make the next Star Trek movie something more akin to District 9, I could see the Horta working as the story was originally envisioned.
That's the big question with the new Star Trek Movies right now, what do they want to be?