Spice Girls (good pop songs....always originals...the biggest girl group of all time but not the best in terms of talent but a great pop act all the same)
Sugababes v1 (had an urban edge, very fresh sound)
Bananarama (consistant in the charts...did well in the US too!)
Sugababes v2 (more commercially successful than v1...but v1 are still my preferred line up)
Little Mix if managed correctly will be right up there....they should be busy releasing song after song constantly for 2-3 years and build up a repertoire (which is what Leona should have done) But Cowell isn't the best at handling artists....he specializes in dealing with puppets and clowns

Will Young (his best winner...which wasn't the result he wanted)got away from him quickly and is still around....Leona has a great voice but currently is stiffed....the other winners have faded away....I hope this doesn't happen to Little Mix....I would hope Cowell left these to someone else to guide the way.
Thought I'd mention Eternal released some good early stuff
The Bangles I think are the very best girl group all time

(they had it all, some top songs, possibly the sexiest female in pop as their lead singer she was hot in the 80's and she still is now in her fifties....they played instruments, harmonised well and had a hand in writing most of their songs)