From Asus AC3200 (6x Antennas) is the lastest but is it the greatests? That will depend on your network needs. That has an extra 5ghz anntenna to offload traffic. If have alot of device then maybe consider it.
Asus AC87u ( 4x antennas)is suppose to be a MU-MIMO antenna designed router. Currently that function is disable. I guess you can call MU-MIMO the lastest and maybe the greatest. MU-MIMO is suppose to be at better handling multiple users at the same time and elminate lag by broadcasting simultaneously to all users. You'll need client devices that supports it. ATM no client device supports it.
Asus 3200 is for the now and Asus 87u is for the future.
There gonna a new router that combines the two. Dlink AC5300 announce theirs and it will have 8x antennas.
If you don't have a lot of users or don't planning upgrade devices for awhile consider Asus 68u or P.
It has the best range and the firmware is more develop. The downside to that router is the USB 3.0 performance is not good, by default it's set to 2.0 to reduce wireless interference with the 2.4ghz band.
Other than that the router is very good. It has the newer software features(Ai protect/Adap QoS) of the newer routers.
http://www.asus.com/event/2014/network/ASUS_AC_family_chart/solutions.html