Steam is the best but isn't so good for sport, at least I've found.
www.gamersgate.co.uk is pretty great for deals and scope and the sheer number of games. There isn't a client for it like there is for Steam and Origin.
Also www.humblebundle.com have great game bundles, and their store is pretty good for deals (not quite Summer Steam discounts, but often better prices during the rest of the year).