May I nominate the British GT Championship?
I saw 3 races in this series last year (Silverstone, Donington and Snetterton) and enjoyed everyone of them.
There is such a diversity of proper GT cars (ie. ones which you can actually buy if you've got the money), much more than the 'international' series, the list published on
SportscarWorld with prospective entries for 2000 includes a Ferrari F40 and F360, Porsche 993 and 996, a Nissan Skyline, a Ginetta G33, Vipers, a Corvette, various Marcos, Listers and a couple of TVRs (including the Speed 12).
Finally, the races are shown on terestial tv in the UK (Channel 4) so you can watch all the action at home and keep up with what is happening.