The "best light weight rifle" is whatever you have in your hands when the best opportunity on the animal of a lifetime presents itself.
Aside from that, a Kimber 84M MT with Lilja's Modified 84M barrel contour (.617" @ 22") chambered to the cartridge of your choice would be my top pick.
That's about as good as it gets IMO. You'll be dollars ahead of about anything else, and have an essentially "perfect" lightweight.
