Since I've probably listed my favourite bands loads I'll use your format I usually tend to like bands more than solo singers, I'm especially picky with female solo singers and I don't really have any favourites, except Regina Spektor and Lily Allen are pretty cool. I like girls in bands when they sound cute ^_^ I like a lot of mixed girl/boy singing bands though.

So,

FAVE (bands with mostly) FEMALE (leads):
1) Architecture in Helsinki
2) The Go! Team (<3 ninja)
3) Tilly and the Wall
4) You say party! We say die!
5) Saturday Looks Good To Me (some of the songs might be only the guy but whatever :P)

FAVE MALE
1) Ed Harcourt, love this guy
2) The Mountain Goats (John Darnielle alone I believe)
3) Mr Scruff <3
4) Get Cape. Wear Cape. Fly
5) James Figurine

FAVE BAND: (at the moment)
1) Suburban Kids With Biblical Names
2) The Boy Least Likely To
3) I'm From Barcelona
4) My Latest Novel
5) The Electric Soft Parade