Well, this poll has been done before and, as an avid wingnut, I must say there are no truly ugly aircraft, unorthodox perhaps but not ugly.
However, the afore mentioned Beluga is a real beauty in my book (no, thats not sarcasm) especially when you look at the Super Guppy it replaced.
For me, the top 5 list of less than aesthetically pleasing aircraft goes to this effect:
1: Shorts Seamew
2: Fairey Gannet
3: Super Guppy
4: Grumman C-2 Greyhound
5: Republic F-105 Thunderchief
One aircraft that, to my mind, looks so bizarre you just have to love it is the Edgely/Brooklands Optica. A bit of a rarity as it prooved quite sales proof . Its sort of like the VW Beetle with wings, threw a lot of design rules out the window and still came out with an eye catcher.
you can find photos of the optica on www.airliners.net