It depends on what I'm trying to buy. There are a lot of really obscure items that you aren't going to find on Northern Brewer or MoreBeer (MoreBeer is my go to for typical items like corny lids, disconnects, poppets, etc. primarily because they are very close to where I live).
If I want, say, a stainless steel pre-fabricated hop spider, I'm going to have to search around for it, probably by asking on this site or other homebrew sites.
Hops I find wherever has the most recent crop of what I'm looking for in the proper size and storage method. Until you get really into the hobby to the point of needing obscure items, the bigger box sites like Midwest, Austin, Northern Brewer and MoreBeer are all pretty much the same to me in terms of service, price and selection.