What, no Enterprise-J choice ? Well, I can't vote, then ! And even then, it's just the least appealing to me. No Enterprise is ugly, in my opinion.
It was a tough call. I hate the NX-Class design, but I had to vote for the JJprise. Its too much of a 'super ship', travels anywhere in seconds, a massive array of weapons, does anything and everything, but still manages to get its ass handed to it.
The OP asks for the UGLIEST Enterprise, not the one with the tech or story role you don't like.