There is no such thing as "the best" item, weapon, armor or anything else. All depends on many factors, like game style, enemies you shoot at, char skills etc.
I like P90c, it can use both JHP and AP ammo, has deadly burst mode from close range, attack costs low action points and ammo is cheap. However if you want to cripple/knockout your enemy, use .223 Pistol or Sniper Rifle (or, if can't afford it, Hunting Rifle) - I was falling on the ground because of those pretty often :>