I like the SCAR, but prefer the ACR. I finished the 2500 kill prestige challenge for the ACR on my 2nd prestige. I'm about to start my 5th prestige, and no other guns are within 600 kills of the 2500 kill challenge. I think I have about 4 rifles sitting at 1500-1800 kills, though.
Also, I started using the UMP this prestige, and it's not too shabby. It takes a while to get used to, but it's pretty nice. My class is pretty close to Kabob's, silenced UMP w/Cold Blooded or Hardline. Backup is a Magnum w/Tac Knife. Nasty, nasty class.