It all depends on your Flight Style. I can say my Favorite Gunship is the Bottlenose, but that doens't mean it's the Best Gunship around. Still, I feel comfortable with it because it's the Smallest and Fastest Gunship around, and I never really liked slow ships.
But if you ask me, considering the performance, Tridente is one of the best boats around. It's a "Gunship" with decent maneuverability, but a 500k powercore and 8 Turrets... Kinda OP, but pretty good. (It can also fire all the 8 Turrets at top corners of the screen, which makes it kinda scary in right hands)
Liberty Gunboat <3 It's slime profile is fantastic and surprisingly (seeing as it has a forward gun) when another gunboat is on your back with a little bit of skill you can easily gain the upper hand. If you just go into thrust and turretview and fire at the enemy behind you while at the same time randomly strafe, thanks to the shape of the thing they barely hit you.