I only own Battlefront II out of all the PSP Star wars games but here is what I know about the rest of them:
-ES is more or less the 'finished' version Free Radicals Battlefront III before that was cancelled; the game has stream-less ground-to-air combat. You can't aim your weapons, as it is tied to a lock on mechanic, but it overall looks like a solid game.
-No idea about the other Battlefront PSP game but it should be a good game
-Battlefront II is a great port of the PS2 version; very stable frame-rate, good to okay visuals, great music, and rock solid controls thanks to the Vita's right stick (mapping the face buttons to the right stick).
-Force Unleashed PSP should be a fun action game, as it is a mix between the Wii version of Force Unleashed and its own thing through 'flash back' missions feature scenes from the movies. Don't own it, but it should be a great time if it is anything like the original game
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-Lego Star Wars III is likely the best Lego game you can play on your Vita, next to Lego Batman 2, as both games are classic lego games; no open-world gameplay but fun, tight level design mixed with some great moments here and their .
-MGS Peace Walker should be a great game, but the PSP version has frame rate issues, so pick up the PS3 version of the game if that is on sale. I would recommend it with a bigger response.....but I am not a huge MGS fan sadly :l.