Personally, I would've discounted Trekkie Monster, purely because I'd want to keep him to one side for Top Five West End Puppets (although I wouldn't stick just to the West End, and Joey from 'War Horse' shoves all the others into the shade as far as I'm concerned). :3 I think... despite the fact it's worn, I'd probably include the Giraffes in the Puppets list as well. There aren't that many puppets on London stages, but the ones that are there are simply phenomenal.
I was thinking it'd be quite nice to include a play or two. But I can only seem to remember a couple of play costumes right now. The one in my avatar (one scene only during the RSC's 'The Tempest'), which I absolutely adore but think is more impactful than beautiful. I genuinely wasn't expecting a skeletal, blood-spattered harpy to pop out of a seal carcass at that point, so it was pretty damn awesome when he did. :3 I also remember the horses from 'Equus'; again though, I think it's more about impact and performance than prettiness. Although the horse-masks probably count towards it, and there ain't nothing wrong with clothing foxy men in skin-tight tops.

(And I can remember the monstrosities in 'Romeo & Juliet', but I wasn't as enamoured of those as certain critics were. XD)
I might have to have a bash at a list myself. We're all different people, so it's perfectly natural for us all to have different lists. :3 What criteria do you recommend using? Sticking to musicals, or encompassing musicals *and* plays if there are pretty enough costumes in both? Just stuff that's currently running in the West End, or can I think about shows that have left us as well? Strictly the West End, or can I break out into the National, Menier, etc? Set me boundaries! I want to play!