We can't say for sure 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.
In self-proclaimed alpha state, FPS Terminator's creator nails the most important aspects of Terminator killing. Namely, the player will kill Terminators, Hunter Killers and more Terminators 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 Terminators, but they really nailed the Terminator laser sounds.
You can download the alpha demo from ModDB or simply watch the mod in action in video form.
FPS Terminator [ModDB]