For high gain? EL34s all the way.
That's just my preference. I also like 6550s, KT77s, and KT88s.
I think those types of tube all have a smoother break-up than 6L6s do. I may have it wrong, but I feel like the waveforms that a 6L6 produces under duress can sound choppy and, well, broken up. It's kind of a low end and low-mids thing. EL34s tend to get more raw in the upper mids but stay tighter in the low end.