Bundles, GMG, Amazon, and occasionally, Gamersgate. They're pretty overlooked, because they don't seem to ever have the best list price on anything, but stack a coupon with something that's on sale and its a great price.

From Mother RUSSIA!

russian friends, plati.ru, traders, amazon, bundles

G2a.com and Gerhard and gmg

I buy my games from Humble Bundle, CD Keys 4 Less and Steam(Rarely)

Steam and GOG mostly and few ones in Origin too. And a few Steamworks games through GreenManGaming, Gamersgate and Nuuvem

The Russians!

The Humble Store and the Bundle Stars store are also very nice and also do lots of deals, even for major games. I bought quite a few games from there.

In order of how often I use them:

• 1 source is traders for me. From pre-orders to sales, almost nothing beats trading TF2 keys for games with someone from another region (i.e. Russians, but also Brazilians, Malaysians and Filipinos now too). Only time I buy direct from Steam now is if its region locked or cheaper to just buy myself.

• Bundle sites. Ive gotten at least 1 bundle from all the big bundle sites at this point.
• Amazon used to be a big source, but their sales have dropped dramatically in the last 3-4 months. They had an event sale a few weeks ago, but I didnt buy anything.
• Greenmangaming and GamersGate are about equal. I did pre-order NBA 2K15 on GMG as I got a good deal on it with a coupon and I got the key 2 days ago (its out on Tuesday) so pretty decent experience for my first GMG pre-order.
• Origin. I had a promo code for FIFA 15 as Im a Red Bulls season ticket holder, so I got it direct from there. Its the only game Ive gotten direct from Origin. I'd gladly get NHL and Madden if they put them on the PC, but no.

Still have yet to buy from Nuuvem or any key seller site like Kinguin, but Im sure I will at some point.