I've been getting all my guitar strings from Webstrings.com for about five years. I've tried several different brands, mostly D'Addario and Ernie Ball, but I'm not a connoisseur or a snob about it. I've never had any brand preference because I've never noticed a difference.
I can't remember how I learned about Webstrings, but they've always had the best prices, plus bulk discounts. They have a wide array of different types of strings: bass, mandolin, nylon, etc. And you can find just about any gauge you'd ever want. (The Med/Heavy .011-.056 will rip you apart.) I'm currently using seven different sets on my guitars right now: .009s on my Les Paul, .010s on one Strat, .011s on the other, flat-wound .012s on my Epiphone, 80/20 Bronze on my steel string, and I forget what's on my P-Bass and nylon string.
The only difference I've noticed between these and other brands is that Webstrings seem to break less often. Other than that, the price is nice, so check them out.