I've use the ones below
Brewers Best - always turned out well for me
Coopers - also turned out well
Munton - I swear I followed the instruction well and the kits didn't turn out bad but not awesome either.
I have recently tried 2 or the Morebeer kits. The Scottish Export 80 Schilling and the ESB. The seem like really nice kits but the instructions could use some clarification. If I hadn't already brewed 8-10 batches or soo I would've been way more confused. My next order will be from Austin Home brew to give them a shot. The other thing I am doing is making recipes.
I'm one of those brewers who's been doing it for a long time but refuses to do the all grain thing. So I've probably done over 100 extract kits. I have always gotten my kits from either the LHBS, Morebeer and or AHS.
Morebeer and Austin Homebrew make awesome kits. Every kit includes all the malt extracts you need to make a 5 gallon batch. No need for adding crappy adjuncts like table sugar. Also they all include the appropriate steeping grains and hops to produce the proper style of beer. No using dark mystery extracts or funky pre hopped extracts. These are top quality beer kits that could easily place in a competition. The only limits on these kits are your abilities as a brewer.