The weapon that packs the most punch (and is easy to use) is the rocket launcher. The weapon that works best on the fly is the DMR. Cuz I said so.
yes the snipe should have been put up here. it is by far the best gun in any halo. in the hands of a good player it can be used in all three ranges.
The energy sword might work. After all you can do unique assassinations with the sword compared to the other weapons (poor grav hammer, golf club, spiker).
Except the sword isn't an option, and I hate the sword assassination. I prefer jumping up and bashing the person's head in. It's quick enough so I can still move on and get a double kill.