I say rocket launcher. Never fails when I lock onto an unlucky player that transformed without knowing that he/she has sealed his/her own fate. It also has the raw fire power AND splash damage I need. Plus in multiplayer, I don't think ammo is an issue since there are almost always ammo clips lying around.
Pulsar Cannon, I didn't use the Missile Launchers in the WFC Multiplayer and I don't use them in FOC Multiplayer.