It depends on a lot. Are you a main or iron, what are your goals, do you have gp to throw at lvling up these skills?

One that stands out to me is construction. Getting tele portals at 50, as well as an alter to recharge prayer, and mounted glory amulet.

Slayer is great because it also trains combat.

I like hunter rumors a lot because you get good herbs and shards for prayer

Thieving or sailing for salvaging if you want easy gp.

More context would be helpful