What sort of running do you want to do with it? And is there a chance you'll want different voltages later?
I use a 1515 2.5d (1650kv) on 5s in my truggy. It's good for racing with the right gearing, and it's good for fairly fast bashing with a bigger pinion. Temps are pretty good (usually less than 120f, unless it's a 100f day). I run it on 6s as well sometimes for bashing and it works well. There's still a fair bit of room to go up in voltage for a few speed runs one day.
This is it in my e-maxx a while ago. I could do ~30 minutes of driving like that and not worry about temps.
http://www.youtube.com/watch?v=ZMZeTJFA2f4
It's the only motor I've used on 5s, so I can't say if it's the best for you or not, but it is a setup I'm very happy with.