if you like super cars, f1 and dtm tamiya is prolly tops. for muscle cars and classic american cars revell and amt both produced some fine kits, amt also had some of the best pickup truck kits, for big rigs italeri has the edge in my opinion. moebius is still a relatively young company but their kits are top notch.
like whats been said before; it depends on what subject matter you're building in as to who has the best kits.