The reason we have so many different answers here is that maneuverability can be defined in at least three ways.
1. Good Dials-- Having many different maneuvers to choose from (and the difficulty of those maneuvers).
2. Repositioning-- Boost and Barrel roll, and preferably the ability to do both in one turn. Also Advanced sensors or BB-8 or Sabine shuttle to do these before moving.
3. Changing Maneuvers-- Here we get Navigator, Stay On Target, and to a lesser extent the YV-666 only Maneuvering Fins.
There is also the SLAM. That is K-Wing only, and is its own case.
I might say the TIE defender has the best maneuver choices, either the A wing or the TIE Interceptor has the best greens. Jake Farrel, PtL Interceptors and PtL StarVipers have the possibility of double post-dial re-positioning.
StarVipers are probably my bet for most maneuverable, due to advanced sensors, boost, and barrel roll, to go with their dial. However, the lack of greens means they are super maneuverable every other turn. PtL Interceptors can't take Advanced Sensors, but can do it every turn.
An Honorable Mention goes to 4-LOM with Stay On Target. As long as someone is in range one at the end of turn, he can pass them his stress and change his maneuver every turn. He can also get Advanced Sensors and the Mist Hunter Title do do a barrel roll before changing the move to a red.
Edited by AEIllingworth