My vote would be to Formula Classic. Im a fan of the this cars and never had the chance to race them in a League.
Would be in also to a League on the Retros, and the lower Formulas.
Make no mistake...F V12, F Reiza, F Extreme...all are great Formula 1 Cars and great to drive.
But we are SimRacing drivers....unfortunately we have to deal with pings, latency and all this stuff. The faster the car is, worse the problem becomes...and usually the race is all but realistic on those cars. Its not good. That´s what I think, anyway.
On this sense, the Formula Classis is pretty much the limit where I can live with in a Online race.