Hands down, the Monk. It took a class that was fun, but difficult to use effectively, and took it to 11.
Weirdly, though, the Way of Mercy (which was a great subclass in 14) isn't a major beneficiary other than getting standard Monk improvements. Open Hand and Shadow are probably the ones I would look at.
(They really need to make a Kensai that plays well with the new monk rules; as it is, it's nearly impossible to make a monk that isn't pure unarmed combat)
Otherwise, I think that the changes to the Warlock are really interesting. By moving Pacts to invocations, and increasing and powering up the invocations, they have made the Warlock much more flexible. The actual "subclasses" don't really stand out to me yet, so much as the main chassis.
At some point, I might want to give the new illusionist a spin. But that would require me to go full caster.
Never go full caster.....