I think the recent development of the 6.5 Grendel is the best all around cartridge for the AR15 platform. Other favorites are the new big bores made for the AR15 such as the .50 Beowulf, .458 SOCOM, and .45 Bushmaster.
What is your opinion on the best alternative to 5.56 in the AR15 platform?
Some research links if you don't know much about other calibers:
http://en.wikipedia.org/wiki/6.5_Grendel
http://en.wikipedia.org/wiki/.50_Beowulf
http://www.ar15.com
http://www.65grendel.com/forum
Here is my 6.5 Grendel (also shown on wikipedia):