My answer probably isn't what you're looking for but:
With the Loadouts system, it's super easy to switch between disciplines on the fly, so I don't really stick with just one (back in the day, I leveled as combat, but that was years ago so the experience is different now)
As far as easiest to learn well for endgame in my opinion:
1) Concentration (fixed rotation, easy)
2) Watchman (hybrid priority/rotation, keep cauterize up, use Zen for Melt)
3) Combat (Precision window priority, resource management can be tricky, squeezing out equivalent damage can be hard)
When I roll my Marauder, I plan to level primarily as Anhilation (Watchman) for the Berserk/Rend combo (Zen/Melt combo). maybe the self healing will be fun, who knows!
As far as fun, I still really enjoy Combat, with its alacrity and speed boosts, extra attacks, and the challenge of aligning precision bursts well.
Then watchman is fun with the heals and dot spreading
Concentration can be fun for jumping around and lading big hits, but it's also so similar to Focus (Guardian) that I can get bored with it.
Ultimately, all of the above is just my take, hope it helps!