When I'm in a nostalgic mood,
I sometimes use the ClassicE theme,
originally by raster but now maintained by vendu.
This theme was cool in the beginning, and it still is.
Actually, I think this will always be
"one of the coolies". ;^)

When I've got a blue day,
I use the Era theme by Dave Henderson (a.k.a. epoch).
I think this is a real cool theme because of the
straight design and windowborders.
BTW: the screenshot is real blurry and shitty, sorry 'bout that...
I'll probably fix it later....(probably? yeah right!)

When I've got a green day,
I use the Neuromancer theme by Nostromo.
This is a real spacy kind of theme,
like you see in the movies.
Really cool, you should definately check it out!

When I've got some sort of an artistic day (???),
I might just use the Aliens theme by Wrex Allen.
A great (but big) theme with a lot of work put into the
making of the window-borders.

But!

Most of the time I use a (slightely modified) NoGrad theme by Dmitry Yaitskov (a.k.a DimaY).
I think this theme is better than the other themes in as well speed as design.
It has real simple windowborders and buttons, so it's fast, but DimaY has also managed to let them look good: the perfect combination if you ask me!
Cool theme!