[Lil' Lavery]

In terms of pure maneuverability, mecanum is the best, as it can travel in any planar direction, change direction almost instantly, and can conquer inclines easier than a holonomic drive. Holonomic and ball drives can also travel in any planar direction and change direction almost instantly. Swerve drives can also travel in any planar direction, but they recquire time (often very small though) to change the orientation of their wheels. "Crab drives" (a variant of swerve drives where all the wheels are linked together) are the simplest swerve drive, but recquire more power (as the have to overcome more scrub) to move in curves. Afterwards comes the plethora of different scrub (tank/skid) steer systems.
4WD, 6WD, 2WD (and 2WD+caster), and many others all fall into this category. While some may recommend the caster system, I would advise against it. From my personal preferance, while quite maneuvable, the lack of control on the casters can cause more problems that it solves. It significantly reduces your control over your bot, and therefor reduces the amount of accuracy you can drive with. Momentum can often cause the rear end (assuming your casters are in your rear) of your bot to swing further than you want it to. It also makes you a relatively easy target to push (and incredibly easy to rotate out of position).
The "best buy" for maneuverability (in my opinion) would be a 4WD with 2 omni-wheels.