Chefs flavours have the best range I think and some flavours that are hard to get elsewhere.
Vapable do free shipping over £10 which is an amazing deal really. You can get 3 more bottles of flavouring with the money you save on postage. They have a pretty decent range, but not quite as good as chefs.
Never used rainbow, but was very tempted to try out their bacon flavour at one point. Sounds well dirty.