I have to go with the Nieuports. They have an elegance which I think is unequaled by any other design.
The Fokker Dr1 is a close second -
As much as I love the Dva - I think it's looks are compromised by the top wing being mounted so close to the fuse - gives it a squashed appearance.
And the SE5a - lots of charm, but brutish is the word
How about ugliest?
my choice, Avro 504 - just can't abide a skinny plane
Usual disclaimer about beauty being in the eye of the beholder

Mark