I know many who prefer UMP, but why? Even though damage is pretty good, it generally lacks in everything else. It's sway effect(when you turn, then stop, it keeps turning for a milli-sec), is rather too much for me. I therefore nominate Vector. Average to rather good damage, extremely fast rate of fire(esp. awesome if you add steady aim to it), and okay recoil. Do NOT recommend ACOG sight with it; stick with iron sights. The recoil is hard to handle if you attach ACOG; without, it's extremely easy.
Vote: VECTOR
ACR is a better choice though.