File1290285872.jpg-(179 KB, 563x1688, 1288896977167.jpg)
I don't know, I suppose they all have different qualities, Gilda is the coolest of them all fitting the whole bad girl stereotype, Trixy was just a bit of a bitch who only knows a few impressive tricks. I'm surprised it didn't occur to any of them that Trixy's magic specialised into her namesake; she's good at tricks which look impressive but aren't very practical.
Nightmare Moon is the only real villain at the moment, more powerful then the cast but arrogance lead to her defeat. Nightmare Moon was the over dramatic typical villain, theatrics, evil laugh, the works.
I'd probably say I prefer NightMARE Moon the most, partly for the pun, partly for the theatrics and partly because Luna is adorable after. You also get the idea that perhaps something else attributed to Luna's evil persona beyond just jealous rage at her Sister. Seems like someone corrupted her which Twilight and the gang removed with the elements of harmony. The elements may actually be highly effective in combating whatever that is, and not Luna specifically. Basically they are the power of friendship used to combat the corrupting force of whatever it is.