I will suggest a couple, and I am sure others have more to add.
6AV6 (and 6AT6 and 6AQ6) These are easy to mod, just cut off pins 5 and 6 and use the EF92 setting. The best tubes are those with a box-like structure (a box getter) on top of the mica:
6HM5 / 6HA5 / EC90
No mods required. Use the EF95 setting. You want to avoid the short squat tubes that look like the typical 6AK5,