Most people in our club who fly them use a .91 four stroke.
In two strokes; most .61, .75, and .91s are physically the same size and weight and fit the same mount; so you have lots of choices.
Every brand has fans and detractors, so asking which is best just gets tons of opinions from people you do not know.
I've read on RCU that Tower engines are notorious for air leaks; then the writer gets flamed by happy Tower owners who haven't had any issues.