A large air compressor is the #1 most important tool. Followed by air tools such as sanders, grinders, spray guns and FILTERS/AIR DRYERS and air regulators.
Then you need power tools. A 240v mig welder is almost priceless, however a 120v/135a mig will do just fine. (Plasma is always a nice add on as well.) Then you need a good stud gun and slide hammer if you want to do dent repair. And a good, lightweight polisher(s).
As for hand tools, a good cordless 1/4" impact with sockets, torx and hex sockets, wobble extensions, and u-joints will be your most used tools. Followed by general hand tools such as sockets, ratchets, wrenches etc. Ratcheting wrenches are very handy when working with hard to reach body panel bolts.
Then you need little specialty tools such as trim removal tools, easer wheel, interface pads, sanding blocks (durablock), etc.
Auto body is a very preference based profession.