We can't say with total certainty whether FPS Terminator, a fan-made mod built on the Unreal Engine, will be the best, but it's safe to assume that it's better than the most recent entry, Terminator Salvation. Sorry, GRIN!
In self-proclaimed alpha state, FPS Terminator's creator nails the most important aspects of Terminator killing. Namely, the player will kill T-800s, Hunter Killers and even more Terminator variations amid a bleak futuristic landscape. Yes, it's "an early work in progress" and a "proof of concept" and it looks a hell of a lot like a beta version of Killzone 2, only with killer robots with glowing eyes, but they really nailed the Terminator laser sounds.
This single-player Terminator themed mod is being developed by Kevin M. Bryant, Ben Sterry and Thom Maggs, an Unreal Engine 3 mod that is planned to span nine levels.
You can download the alpha demo from ModDB or simply watch the mod in action in video form.
FPS Terminator [ModDB]