That's an interesting dilemma. What would you rate as being the worst finale, Nemesis or These Are The Voyages?
TATV simply because it disappointed me the most. After Manny Coto took over the reigns for the last season I was really enjoying Enterprise. Finally the show was in a direction that I thought it always should have been. Then Braga decides to do his "valentine" and we get a ridiculous episode where two actors try to shoehorn themselves into an old TNG episode and it just didn't work. Take Riker,Troi and the holodeck out of the episode and it's at least passable. A finale should focus on the cast that we've come to know, not on guest stars.
Nemesis really didn't bother me as much as it did the fans. I would have preferred Picard's Nemesis to be a Romulan or maybe even a Reman,but Hardy did a good job with his role.