Mine would have to be:  I Think it's one of her most memorable costume. 
 
But others are not so appealing: Does anyone else think that this looks like Jean's first costume?

Out of the history of the X-Men, Kitty has had some god-awful outfits.  What were they thinking with the costumes she had as Ariel or Sprite.  I didn't care for her brief all yellow-green outfit and the all blue one was just barely a step up from that.  I did like her later take on the standard X-Men uniform and the outfit she wore in X-Treme was great.   The problem with Kitty is she's one of the few characters that has actually managed to age appropriately in comics.  She's started out as a teenage girl and she's now a young woman.  I've found that artists tend to render her still as a teen and any costume she has can turn against her if not drawn with age in mind.  The character has evolved, her outfits should too.

" Out of the history of the X-Men, Kitty has had some god-awful outfits.  What were they thinking with the costumes she had as Ariel or Sprite.  I didn't care for her brief all yellow-green outfit and the all blue one was just barely a step up from that.  I did like her later take on the standard X-Men uniform and the outfit she wore in X-Treme was great.   The problem with Kitty is she's one of the few characters that has actually managed to age appropriately in comics.  She's started out as a teenage girl and she's now a young woman.  I've found that artists tend to render her still as a teen and any costume she has can turn against her if not drawn with age in mind.  The character has evolved, her outfits should too. "

This has been done before:
http://www.comicvine.com/kitty-pryde/29-3548/fav-costume/92-21051/#8

They just now need to make a totally different costume for her now that she's coming back. 

I like this one

   
love the classic blue on blue Shadowcat look