It really does depend what you want.
As I want to drive my TR7 and compete in rallies it's Club Triumph.
I am also not such a purist that TR7s are the only Triumph I am interested in so CT scores again.
Having said that, if you want a good local social scene then as Gordon says, it depends where you are in the country.
I have been in the TR Register but only for one year - I didn't find anything particularly interesting.
TRDC - don't know but I would think if you want a pure TR7 experience and shows etc then that would be the club to go for.
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I wish I could drive a TR7 like Tony Pond
North Yorks/Lancs border
http://raidertr7.blogspot.com/
TR7 Sprint FHC tarmac rally car - Speke built ... Barnoldswick modified - there's a big difference!!!