R80.40 with JHFA102 as a minimum seems to be ok, at least for me. If your running R77.x defo move up to R80.40 with JHFA102, but of course plan and check for any specific hardware requirements, do a clean build!
I've not really used R81 yet so can't comment, but as Dameon states it really depends on features your after. I suspect if you have high end appliances (30 cores plus) then something like dynamic dispatcher is a useful feature which is available on Gateways (not VSX at the moment).
Other features I'm looking forward to seeing in action are : NAT Hit count, accelerated policy push, and multiple policy push at the same time (R81).