If a live-action Teen Titan movie based off the Cartoon Network TV show was being made this is who I would want them to cast.
Robin- Daniel Radcliffe
I picked Daniel Radcliffe because he's shown leadership throughout the whole Harry Potter series. So he is used to being a leader which is exact trait he needs to play Robin
Cyborg- Tyrese Gibson
I chose Tyrese because in Transformers he showed he can play as a wingman. A wingman is what Cyborg is, he is basically the leader after Robin.
Raven- Emma Stone
Ok, so I didn't pick Emma Stone based on if she looks like the character or not, I picked her based on her personality. Stone has that dark tone that Raven has.
Beast Boy- Ryan Reynolds
I know Ryan Reynolds is already Deadpool and Green Lantern but i think this character fits him the best. Beast Boy has the humor of Deadpool and the green of Green Lantern. Personally I think Reynolds is suited better for a comedic role anyways.
Starfire- Amy Adams
In roles like Giselle from Enchanted Adams shows she can play the crazy hyper type. Which is why she is perfect as Starfire.
Slade- Vin Diesel
Because of his role in the Fast and Furious series he's shown he can play the "bad guy". I think he will do fine as Slade.
Terra- Hayden Panettiere
If they do make a Teen Titan movie I'm guessing they would use the Terra story arc in which case Panettiere would be the ideal choice considering she did play a girl with powers in Heroes.
Let me know what you think at the comment section below.