The Jones Underlever is probably the strongest action type because each time you close the lever the barrels are literally screwed tight to the action. H&H and others used this action well into the 20th century for many of their biggest bore double rifles.

However, I'd never call it the "best action type" because it is slow and somewhat cumbersome to operate.

My Joseph Lang 16 bore is a Jones Underlever built in 1866 and still going strong.