Well there are a number of places, and where you will find the most from the game, will definately come down to a personal preference.
As you progress your skills you will find yourself moving around to different spots as well.
My favorite place for working on cutlass is Fort Dundee on Padres. I hang out in the corner, and feast on Veterans, and Officers that spawn, and march into this corner on a regular basis. Regular enough to eliminate them before the next batch, but doesnt leave ya waiting for enemies to respawn.
Im sure there will be a number of other recommendations coming along
