20" Bull Barrel with a 1:9 twist to handle bullets from 45 to 70 grains and a Free Floated Fore End is my vote.
As to whom you should buy from - Good luck right now. Any quality barrel makers out there are backlogged BIG TIME.
Krieger makes the best AR barrels on the market, for my money, but they are about 8 months out on anyone not ordering about 150 of them at a time.
We tried to order 30 in a bulk for some AR kits we wanted to put together and were told it would be late summer, early fall before we could expect them.
Shilen and Pac-Nor are some choices, but they are a little lower on my list personally. Doesn't mean they won't work well for you.
I would check out some of the on line sights and see if anyone is selling a barrel you can pick up without having to get the whole gun. The_Republican had, I believe, a 20" barrel that had like 100 rounds through it and he was selling for a good price, I don't know if it's still for sale or not.
Just some thoughts off the top of my head this morning...
Feel free to PM if you have any questions or need some help with the build.