A "dozen" different sets is something of an exaggeration. It does have a superb movepool and great STAB typings that give it a lot of flexbility, but changing one move or using a slightly different EV spread is not a new set.
So far, sets established as effective are (in no particular order):
- Life Orb (Taunt + 3 Attacks, can run Focus Blast for coverage, can also mess around with things like Substitute)
- Specially Defensive (Roost + Taunt + Toxic + Foul Play)
- Assault Vest
- Choice Scarf? (Have not tested yet, it has some advantages as a Scarfmon but feels outclassed by Xerneas, which covers MMX, Darkrai and almost all dragons in 1 move)
Again, there can be some variation, but all actually viable sets will fall roughly into one of the above categories.
Xerneas can arguably match this:
- Geomancy
- Choice Scarf
- All Out Attacker?
- Cleric? (The latter two sets need to be evaluated to see whether or not they are outclassed, but seem effective and its typing helps to give it a niche)