best is entirely subjective.
do you define best by rate of fire? in which case an arp with a polarstar jack is the best
do you define best by accuracy? in which case a well tuned fusion engine or f2 in a gun that also has sufficiently good components (hop, barrel, reciever alignment and rigidity etc) to take advantage is the best.
do you define best by loudness? a short barrel with a high pressure system is best
do you define best by quietness? a long barreled system with a low flow setup and a big suppressor is best
do you define best by recoil/handling? in that case a converted gbbr or daytonagun kit is best
the list continues, but the moral of the story is the best gun for a given application won't be the best at another application.
hpa isn't a "magic bullet" you don't just drop an hpa kit into a gun and suddenly it'll be amazing and outperform everything on the field, if you want performance in any given direction then just like any other airsoft gun you're going to need to work for it.