Generally, the slower your plane is, the tighter you can turn without blacking out. On the other hand, if you're slow, you'll be covering less distance so it'll be easier to track you.
It also helps to not be weighed down with bombs, rockets, drop tanks, and a lot of internal fuel. Some planes offer different gun loads, and the bigger guns usually weigh more and will affect aircraft turn performance.
To practice loops, you might want to try the more loop-friendly planes, like the FM-2, A6M Zero, Spitfire V, Spitfire IX, and N1K2.