L96A1 and M24 are the best SR's.
L96A1 for its one hit KO and M24 for it's deadly accuracy. If you're quick to aim, go with the M24, if you just want to take people out fast, go with the L9.
I personally don't like the semi-auto sniper rifles. I find them cheap and it doesn't take much skill to use them (since if you miss, you get another shot right away).