I voted 600...
I have killed a few deer clean at 850, but have decided I was pushing my luck...you know how it is...getting older and wiser and all that. The longest kill shot I've ever made was on a coyote at 1,019 yards...coyotes are different than deer in my mind, if I can see them...I'll shoot them!
I no longer think a 308 is appropriate for deer at that range...it worked fine but any number of things could have caused it to end badly...I was lucky enough to figure that out on my own without having to learn it the hard way.
These days I'm shooting a 30-06...and I still limit myself to 600...I'm confident in my abilities at longer ranges and I have a place to practice out to 1,200 yards...but I don't have the right rifle for that range anymore, the 300 WM needs rebarreled, and I'm in no hurry to have it done...kicks too much to suit me anyway