Best quality: Honestly, I haven't run into an online dealer that I would consider "poor" quality. I've had good experiences with liveaquaria, reefhotspot, marinedepotlive, and saltwaterfish. The only issue I ever had was some sick clowns from marinedepotlive. I called them and they gave me credit on the spot without any issues. That being said, liveaquaria has the best guarantee on their livestock, and having just ordered from there would not hesitate to do so in the future.
Cheapest prices: This one is tricky... It all depends on which fish you're looking for. Some sites are cheaper than others for certain fish and pricier for others. I find that reefhotspot has some of the best prices I've seen on a wide variety of fish. Saltwaterfish has good prices, but man they don't carry a lot of variety. Most of the fish on their list are never in stock.