Depends, what role do you need it for?
If you want a one-shot kill at close range, but as a backup, use a Rough Cut or a Sledgefire (after removing the stock). This is useful for keeping people behind cover, since they know they won't be able to dodge two or three darts just to poke their heads above cover.
Need a good mix of fire rate and range for a primary? Use an Alpha Trooper or a Rampage. I prefer the Alpha Trooper because loading from the bottom makes more sense to me.
Want an effective sidearm that has good fire rate and decent capacity? Get a Strongarm, you can slam-fire it.
Want a good springer overall? Get a Stampede and upgrade it, like Captain Xavier did with his primary, EIR. Yes, technically the Stampede is a springer, even though it takes batteries, because the main power going to the darts is from a plunger and plunger tube, the mechanism is just primed and reset electronically.