Avocado oil is the healthiest option for seasoning a cast iron skillet, and great because it has a very high smoke point. Canola oil and vegetable oil are often recommended, but these are not great options if you are wanting to avoid highly processed oils. For daily use, I use coconut or olive oil even though they have a low smoke point. Since these are the two oils I cook with most frequently and they are more affordable than avocado oil, these are my personal preferences.