DogFoodAdvisor.com is the site Nik is probably talking about. They have some great information.
They also have a long running thread on big dog diet & nutrition with some interesting commentary:
http://www.dogfoodadvisor.com/forums/topic/large-and-giant-breed-puppy-nutrition/
The main thing is to make sure the calcium is 1.5% or less, and that phosphorus is balanced with calcium or a little less (~1.2-1.5%). Most big or giant formulas are on board with that these days.
I'd opt for a grain-free variety, but often they rely on pea or soy protein, which might not be the best, either. Read the ingredient list and make sure you like what you see.
Acana, Orijen, Now-Fresh, Fromm, Solid Gold and many others are good quality foods.
We fed Denna Solid Gold Wolf Cub until she was about 6 months old, then moved her to raw ("whole prey model raw"). I love feeding fresh food and having control over the ingredients in her dish, but it does take some planning and, in our case, an extra freezer.