Rayquaza or Togekiss
I highly recommend using rayquaza because:
1) His base stat total (especially when mega) is unbelievable
2) He's available in ORAS
3) He has a wide move category of types and attack
4) You can make him a physical or special sweeper.
5) Minimal weaknesses
Rayquaza is the best Pokemon for a sweeper because he's an amazing Pokemon and looks really cool when shiny.
If you don't want to choose a legendary, choose togekiss because:
1) He would make a great special sweeper, and provides a great range of defense (to balance off weaknesses)
2) Can learn a wide variety of moves in various types
3) A fairy type that's really good and good against many types.
I would highly recommend rayquaza if it fits your choice, since he can mega and provide an extra item slot with it too - with his outstanding stats. If you can't use legends, choose togekiss because he's a great Pokemon in my opinion and would make a great Pokemon for your team. Plus, their both flying types with suitable abilities.
Your choice! Hope I helped! :)