I'm enjoying the circuits the most. But I've been winning a lot with the McLaren 12C and Ferrari 458 Italia. Started my career off with a few classics though, the 1999 Dodge Viper GTS and 2002 Corvette Z06.
Funny that you mention the rain. I was doing a race at Brands Hatch in the wet and it struck me that I don't recall a GP being run at Brands in wet conditions. Apparently the 1966 race started off Wet but that was about it. And yeah, the controller feedback is incredible. Lot of aquaplaning through the puddles.
Some of the other cars I've been impressed with are the BMW M5, Alfa Romeo 8C Competizione, Aston Martin DBS, and 2013 Hyundai Genesis, seriously
Other thoughts
Driving at old Monza going into Curva Grande full throttle is downright insane.
According to FM6, the top speed of a Formula E car is 139 MPH. I tested it at an Airfield.
Only got a little tease of it, but the BMW Procar seemed incredible.
The Lotus E23 (driven Online) is basically undriveable using a gamepad without all the aids turned on. I assume its quite difficult with a steering wheel as well.
My favorite Tuning trick is to decrease the Tire Pressure by 3-4 lbs. Seems to work pretty good.