KT66 IMO. The 6L6GC is also relaxed but isn't as resolving as the KT66. I currently use Groove Tubes KT66C with a hardness rating of 10 in my McIntosh MC240, and I find them to be excellent.
I've got Mullard EL34's (produced in Russia by New Sensor) in mine now and they sound great. I didn't like the Russian NOS 6P3S-E (I only tried one plate voltage...) or the stock 6L6GC Sovtek's (altho I liked them better'n then the 6P3S-E's).
Thinking about trying the Genalex Gold Lion KT-88 reissues next....