hands down Lynx. Just take the RPM * Max damage to get its DPM. Divide by 60 and you get DPS. Infantry have a 1000 HP theoretical total, 1200 with Nanowave, and 1250-1500 on Heavy assaults depending on load out and reflexes. Any weapon with a Sub 0.7 DPS will usually be a non-retaliatory kill if you get first strike and don't miss. Add 0.7 to your TTK if you are coming out of sprinting.
After using TRAC-Shot, I have to change my statement. The carbine with fastest TTK is TRAC 5 S with underbarrel shotgun and TRAC-Shot's underbarrel shotgun. Underbarrel shotgun's TTK is zero, since it kills instantly, as long as target is close and you hit it just right. Zenith. Very similar, but with couple of quirks: only 30 round mag, no access to SPA, but has access to both advanced laser sight and advanced forward grip (can't be used at the same time, obviously).
