I play a lot of 6L6 Fender gear, but my faves are KT66, 7591, 6CA7, 7581, and 7027's. Just good clean output power, reliable, long service life...a little bit of nonlinearity goes a long way.
6V6s, and 6L6s (Crate stealth/Peavey ultra/Mesa Mark iv)
More 3D sounding to my ears.
EL34/EL84s sound too stiff to me. (Marshall DSL 2000/Carvin Legacy/Mesa F30)
I've had amps with KT66s, el34, 6v6's, and 6l6s- I think I like the el34's the most for gain, but for a good clean fender sound I like 6v6s or 6l6s. Since I tend to play crunchy, el34s are my vote.
it really depends on what you are going for, and what your tastes are. hard hitting? 6550, kt88 and 6L6. punchy and smooth, kt66, el34. squishy, but with character, el84, dependent on design.