I think the best wood for a beginner is the best piece of whatever you have in front of you.
It's hard to beat osage, but straight, clean, smaller diameter elm, ash, hackberry, or chinaberry would be excellent to start with. I think making flatbows of those woods is very straightforward.
But, you got a lot of good options. If it was me, I'd want to be working on a bow right now, instead of working on learning backings and chasing rings. But, I'd be doing those NEXT week, honestly.