Had to go through my Gmail history for this.
Steam is by far my most used DD service. I have close to 200 games through them. The only reason I will use another DD service over steam is if the game is not available or if the sale is really good.
After Steam, GoG and D2D are my most used services. I have over a dozen games on each. GoG is probably my second favorite DD site. Tons of DD exclusives, great pricing, and a lot of bonus materials thrown into their releases. I use D2D mostly for the sales. Every once in awhile they will have a game not available on Steam (Sims 3, Neverwinter Nights 2). They are also better than Steam for offering discounts on Brand new titles because they will sometimes offer generic coupons (like 25% off 49.99 or more) that are applicable to many or all games on the service.
I also checked off Impulse and Gamersgate, although I rarely use either. I have around 5 games from each. On Impulse I have Dragon Age Origins and a bunch of Stardock titles. DA:O was purchased there because I had a coupon for 20% off at launch. However, I was really disappointed with how the launch was handled (no pre-download, and the game didn't even work at first. Had to find a replacement file buried in their forum). Probably won't buy new from there in the future. Some of their sales are alright, but typically they offer 20-50% off when other services routinely offer 33-80% off. Gamergate has a lot of DD exclusives, and I would like to support them more, but their download speeds are horrendous (at least they were last time I used the service about 10 months ago).
Finally I checked off the other category. I have used the EA store, Blizzard's store, Telltale games' store and a few other generic DD services in the past year. Typically for a single purchase.