All of the cartridges you mention would be ideal for deer at all reasonable ranges...
The 6.8 is a nice cartridge. However, its got a slight bullet problem in that most bullets in that specific diameter are made for 270 Win. Any bullet that really works well in 6.8 is a bullet designed specifically for the 6.8.
The Grendel is a 6.5mm, and unlike the 6.8 SPC, there are already a whole bunch of modest, moderate 6.5mm cartridges. Lots of 6.5 bullets are therefore made for things like the 6.5 creed more, 6.5 Swede, 260 Rem, etc. And with the Creedmore rally catching on, you'll likely see more and more really good 6.5 mm bullets. The Grendel with a Ballistic Tip or SST would work very very well on deer. In addition, I suspect the Grnedel would be slightly better at longer ranges..
The Creedmore is a awesome cartridge. But you won't get it in an AR15 platform. Its a 308 sized (approximately) case, that won't fit into any regular 5.56 AR15 action.
Personally, I'd go Grendel