6V6 - 6F6 - 6K6 - 7408
The 6V6, 6K6 & 6F6 family of tubes are famous for their excellent tone and mass availability. The 6V6 is the most powerful of the bunch and can be run in place of any 6K6 or 6F6. You can also usually get away with running a 6K6 or 6F6 in place of a 6V6 though you may run into issues with distortion or volume.
Drop-in Substitutes for the 6V6: 6V6GT, 6V6GTA, 5871, 7184, 7408, VT-107A, 6P6 (Russian).