Anything low (1-30), or higher (70-110), ideally infested, but if not, generally larger floors like a 0->15->16 and repeat is pretty good, same with 13(?), though requires more ladders as 10-12 don't really offer anything of worth.
Otherwise skull cavern, harder to kill but they're generally rather plentiful and you're not really wasting your time, as you probably have more monster slayer goals to do there than remaining in the regular mines (at least ones that actually coincide with the floors you'd be farming for slimes)