I spent hours finding one with both a good nature and possibly Hidden Power Fire. The idea was to use this set:
-Hidden Power Fire
Its speed gets doubled with Sunny Day, and Solarbeam can be used without charging up, and Hidden Power Fire gets boosted.
So I got a Modest (+sp.att, -att) one without Hidden Power Fire, and I got a Quiet (+sp.att, -speed) one that did have it. I decided to go with the Quiet one, even with the speed reduction.