Best combination of both, or best of each?
The vanilla Eagle has nearly the best rotationals while also having very good acceleration in all translational axes, making it the most maneuverable, overall. It's not particularly fast though.
The Imperial Eagle can reach the highest usable boost velocities in game, but has more modest rotational and vertical/lateral thruster performance.
The Viper III can reach the highest non-boost velocity in game, comes very close on boost velocity, is tied with the FDL for highest boost acceleration multipler, and has probably the strongest vertical/lateral acceleration. It's rotationals are modest though.
The Imperial Courier has a very good combination of nearly all aspects of speed and maneuverability. It can also retain more of it than most of the above when outfit for something other than raw speed as it's extremely low hull mass gives it some leeway.
The Imperial ships both need Imperial Navy ranks, but this is very easy to get for vessels this low on the scale.
What about the Imperial Fighter? I heard that was the most maneuverable
Arguably true, but it's also not it's own independent ship.