I gotta go with the place where I bought most of my games last year: MicrosoftStore.com.
I had never dealt with them before until I found out about the preorders they were doing for October-November releases. $10 credit off a future purchase, plus 1600 LIVE points, and free shipping. I preordered four, and all of them arrived on launch day, some of them showed tracking as overnight deliveries.
The LIVE points let me reap the rewards of the holiday XBLA sales, and the $10 credits were used up during their amazing Black Friday Weekend sale, when they had deals like XCOM and Dishonored for $25, Forza Horizon for $15, and a slew of older games for $9.99.
I'll be working my way through that backlog for quite some time....maybe until they announce their preorder deal for GTA V this September.