I enjoyed 2 and 4 the best. 5 was the worst (didn't see Nemesis). 1 was pretty good but ran way too long. The franchise is in poor shape now, imo. I don't know if it will survive another bad movie.
I think its Part IV, The Return Home. Always cracks me up seeing Kirk cross the street and the taxi driver says "Watch where your going dumbass." and Kirk says "Well a double dumbass on you!"
Ricardo Montalban is nothing without Tatu next to him yelling: "The plane! The plane!" I gotta go with the first one, ST: MP, because it was like being on an acid trip without actually taking any acid with all those whacked out effects. Granted, there wasn't much for space fights, though. Plus, that chick's bald head and robotic voice tripped me out.