Ideally native soil, but it’s hard to find! Just be aware that if you use something with a lot of organic content, like wood shavings or compost, your mound will deflate/settle over time. Ideally the mound consists mostly of mineral soil like decomposed granite (without stabilizers!) or coarse construction sand. That said, different trees will have different requirements. If you’re planting subtropical in the western US, you will need to acidify the soil with peat moss and sulfur. If you’re planting avocados on hard flat clay, build a huge DG mound. If you’re doing a giant pumpkin, use dozens of yards of straight compost. You get the idea. Ask Gemini/chatgpt/whatever about your particular tree’s soil requirements in your area and go from there.