The invisible stalker, another beautiful example of WotC's descriptivist tendencies.
The chassis: 8 elemental RHD, which are worse than warrior levels in multiple ways. At least there's a few immunities attached. Two slam attacks, which are okay but not great (again, a same-level warrior is better). Bonuses to all stats but charisma are sure nice, though.
Improved Tracking allows for making Spot rather than Survival checks for tracking. Assuming you take Track, I guess it's a minor buff.
Finally there's Natural Invisibility, essentially granting a stalker an improved Greater Invisibility all-day. Unsurprisingly, it's their main selling point, and I must say it is pretty good.
However, note that a stalker does not have Hide as a class skill. There's ways around it, but as they are now most CR 7-9 creatures will spot them half the time. That somewhat hurts versatility.
I think a LA of +0 works best, in the end.