I'd rank:
1. The Wrath of Khan
2. The Voyage Home
2. The Undiscovered Country
3. Reboot
4. The Search for Spock
5. Nemesis
6. First Contact
7. Generations
8. The Final Frontier
9. The Motion Picture
10. Insurrection
Voyage Home and Undiscovered Country are polar opposites so it's hard to rank them, but I'd consider them both close seconds to TWoK.
I know, I'm weird for ranking Nemesis highly. I give it credit for being the only one of them to succeed in replicating the tension of TWoK, even if almost everything else about it is pretty bad. I also like the *concept* of Picard's (potential) greatest adversary being a mirror image of himself, because I think that's essentially what the series built up in his character. It was telegraphed a little too literally though.
That said, everything below Search for Spock for me is the 'bad' half of Trek. I don't really ever feel compelled to watch any of them again, except maybe TFF for a laugh (its one saving grace is that it has some of the best Kirk/Spock/Bones moments in all of Trek -- Also, Scotty + bulkhead = laugh every time). Insurrection is so much worse than the rest that it's painful.