One of the great things about Star Trek is its "rewatchability factor". Most of Star Trek is like sex and pizza, even when it's bad, it's good. But I'm sure that every fan has a list of those episodes/movies that they have no desire to ever revisit.
So here's my "top 10"...
1. Star Trek: Insurrection
2. Enterprise - Dear, Doctor
3. Star Trek - And the Children Shall Lead
4. The Next Generation - Sub-Rosa
5. Voyager - Fair Haven/Spirit Folk
6. The Next Generation - Code of Honor
7. Deep Space Nine - Babel
8. The Next Generation - The Inner Light
9. Enterprise - A Night in Sickbay
10. The Next Generation - Homeward