From best to worst:
#1) Star Trek II: Wrath of Khan. The special effects of the eighties aren't quite up to par, but the best storyline by far.
#2) Star Trek VI: The Undiscovered Country. A fitting sendoff for the original crew.
#3) Star Trek XI: The new kid on the block, I'm still evaluating this one. But I really liked it.
#4) Star Trek IV: The Voyage Home. Save the whales! Lots of fun. Lots of colorful metaphors.
#5) Star Trek Generations
#6) Star Trek III:
#7) Star Trek First Contact
#8) Star Trek I: Slow, the beginning really drags. Did they really need to spend 30 minutes spanning different shots of the Enterprise? This was lame even when it first hit the theaters and the special effects were new and cool.
#9) Star Trek V: Hands down the worst movie by far. What were they thinking? This one stinks so bad I refuse to buy it even with a set.