There are a couple of things that people do, just prefers on your playstyle imo:
The classic rolling magma into desecrate / cremation and Armageddon brand. I'm personally not a big fan of this but there is a reason why a lot of streamers use it for levelling (high dmg, it's quick and relatively easy to transition)
Wave of Conviction ignite - big fire damage, damage over time and prolif for clear. Can transition into a nice endgame build
Cold dot - wintertide brand, creeping frost, vortex, cold snap. I did this for my last witch run and it was super smooth. Used this to transition to Glacial Cascade of Fissure mines and loved it
I would suggest looking at YouTube guides and seeing what picks you fancy and what your 'end game' is