Whatever I have on hand or fit's the dish.
Mostly rapeseed oil, sometimes olive oil. When I first started (before I was able to just guess), I put a drop of water in the pan. As soon as the water started to boil, I added the oil and was ready to go.
Nothing stuck for quite a while. But if it did, sometimes it was enough to wait a bit for everything to release by itself.
Aside from that: butter. Butter does some magic.