Mugello and anything is a great combo. It is my favorite default track.
Monza 66 and the louts 49.
Far as the ring goes i loathe the mclaren gt3 car there, the other gt3 cars handle it much better, but for my money the two best combo are the 312t, and the zonda r, t125 is a kick in the face but some of the zomg factor is lost over the 312.
Hard to pick one favorite... Lotus 2-eleven at Mid-Ohio with chicane; Lotus 49 or Ferrari 312T at Mugello; Lotus Exos 125, either base or Stage 1, at Imola, Nordschleife, or Road America - any of those is spectacular fun!
Oops - just re-read the OP and saw the word "online". I'm still awaiting my RD license, so I don't know if any of those are ever available here. I'm pretty sure not all are in the AC public servers.