Honestly, I'll buy from Beach over B&H/Adorama any day. OK, their site is a bit shoddy, but their prices are low and they ship fast and they ship free. They're also responded to any questioning emails I've sent very quickly.
As for the other two, I prefer Adorama just because their site is better. (Specifically for looking through lenses, instead of B&H's frustrating way of completely segregating the "film" and "digital" lenses.) Of course, Adorama hasn't sent me an enormous photography catalog like B&H did, but on the other hand, they don't send me more general catalogs that go right in the recycle bin like B&H do, either.