My most popular colours just now, are;
Good Gossip : glittery red...very christmassy!
Bella's Vampire : very dark almost black winey/purple!
Glamour Queen: pearly pink (most popular with the more mature client)
Standout : transluscent dark red, great on it's own and brilliant over other colours
Sweet Chocolate : brown with gol shimmer......2 coats of this then Standout gives a gorgeous deep claret colour!
I used to apply 3 coats of Good Gossip to get a really deep glittery colour, but have now found 1 coat of Standout and 1 coat of Good gossip gives the same effect!
For french, sheek white and sheer pink....although Ambience is a shimmery pink and also great for French manicures!
I thinks tastes vary from area to area and depends on your clientel, but I hope this helps in some way.