Great question! I think it really depends on what "best" and "worst" means.

First service, seats, and legroom depends a lot on the airline. The "best" economy seat is usually the exit row, which has much more legroom than most of the worst first class seats. And crappy service can be found in any airline in any class (though perhaps rarely in first).

I'll take the best economy on Asian carriers like SQ, which has a great service even on short legs. On domestic, I'd probably take a worst first over the best economy (i.e., best of the worst).