Hello Andy M.
In general, everything! However, if i had to decide, it would be developing the route of a gas line and of course, unclog drain, because there are always new ways to find strange debris.
From a business standpoint, I love a remodel call or full re-pipe, but I actually enjoying rebuilding faucets. They don’t make any money, but almost nobody does it well anymore and every once in a great while you find something unique or different that can be a challenge.
A clogged drain call is always a profitable venture. With the correct equipment and necessary experience you are going to be able to unclog the drain, make your customer happy , and make money with out spending money.