Well the "latest" is the Geissele Mk15 rail that debuted yesterday...and most aren't impressed with the look...but it is indeed the "latest"
"Greatest"...is a matter of opinion. If expense is an indicator, legacy Geissele SMR's (due to availability) and the Lancer CF are the highest priced on the market at current.
If you want a SEAL clone, pretty much one of 3 things:
- OEM quadrail...cause it's what came on the gun
- Geissele SMR
- Remington RAHG
If it helps, Geissele rails are probably at their highest market value since their release; and Remington RAHG's are at their lowest.