As always, you will see the best benefit when you choose the spec that makes you happiest.  Holy is just my recommendation.  Shadow is a good second contender.  Disc is a far third, in my book.  It’s damage just isn’t as strong as shadow or holy.

• Disc leveling spec:
  A slower, more controlled leveling experience.

   

  • At level 70, spec: 51/7/3
  • 71-73: Divine Fury
  • 74-75: Aspiration
  • 76-77: Grace
  • 78-80: Inspiration
  • End at: 55/13/3

• Holy leveling spec:
  Take mobs down in about 10 seconds, and heal well, too!

   

  • At 70, spec: 14/44/3
  • 71-73: Empowered Healing
  • 74-76: Serendipity
  • 77: Guardian Spirit
  • 78-80: Test of Faith
  • End at: 14/54/3

• Shadow leveling spec:
  Multi-mob killing machine.

   

  • At 70, spec: 10/0/51
  • 71: Twisted Faith
  • 72: Inner Focus
  • 73-75: Meditation
  • 76-77: Mind Melt
  • 78-79: Veiled Shadows
  • 80: Imp. Mind Blast
  • End at: 14/0/57