I've started playing War Thunder, it's a free to play combat flight sim and it's great. As soon as I started playing it, it instantly made me think of the idea of a flight sim that looked and ran as good as this did..
Anything by FSDT or FlyTampa is pretty much guaranteed to be fantastic, the others that have been mentioned are also great; FlightBeam, LatinVFR etc
I love FSDT's KLAX, not only does it look brilliant but I always had pretty smooth framerates too...