Primary choice for standard systems is a refurb from Apple, going via the DiscoverCard site which gives a 5% discount. Custom systems from Apple online, usually using my educational discount if I haven't reached the 1 system/year limit. I've also bought several directly at the Apple store. I frequently take advantage of the free printer deals, usually giving the printers away as gifts. Also the free iPods in the back-to-school promotions.
I've got no reason to buy from anywhere else since there is no tax (in Oregon) to consider and Apple also has free shipping like the others. I also work about a 5-10 minute walk from an Apple Store.
I also work less than 2 miles from Powermax, but they don't have a retail storefront and end up being more expensive than Apple after considering Apple's promotions. Other local possibilities here include Best Buy (which I wouldn't buy from for any reason) and Fry's Electronics. There's also a retail outlet for Powermax called The Mac Store, I believe. Anyway none of them are less expensive than the Apple Store.
I'm a fairly big customer of Amazon and B&H Photo-Video, but no advantage to buy from either of them.