If you're a non-partner, 720p@30fps, 2000kbps, x264 Veryfast.
You have to keep your bitrate low to avoid having your viewers buffer. Twitch released usage metrics a while back showing that 2000kbps is the point at which most people can watch smoothly. Even going to 2500 causes problems. 720p@30 fits nicely into the 2mbps 'budget' as one of the best tradeoff points.
On the up side, you can go to a slower (not the Slower) preset. How low you can go depends on the game you're playing. Start at Veryfast and live stream for 20 minutes, watching your temperatures, CPU load, and throttling. Go down one step at a time and test for another 20m. You want to keep good temperatures, and around 80% CPU load during live gameplay. The slower the preset, the better the compression, and so the better your stream will look.