Gee... this is a fun one.
The best formula to get into it money was not an issue... I don't know... maybe F1!
Ok, really... if money were not an issue then you could focus on why you're in racing in the first place... SPEED and COMPETITION.
SPEED is relative. A close race @ 50 mph is just as much fun as a close race @ 150 mph.
So that leaves COMPETITION. Even the Indy 500 wouldn't be much fun if you were the only one on the track. You need others to compete with. Which formula has the most, really tight competition? I'd have to say Formula Vee. The fields are usually the largest of weekend and the top 5 or 10 can be covered by a blanket!
Oddly enough, it also, by far, the least expensive?! Go figure...
Delta Region, SCCA