For me this gets broken out into two categories. 1) weapons that would look really cool and 2) weapons that would actually be useful.
However powerful the rocket launcher or Spartan Laser would be, they're not terribly cool looking weapons for display and they're not useful at all in everyday life.
So I would say 1) Needler and 2) Energy Sword - easily the coolest looking weapons of the series
And for most useful, either 1) Reach DMR (powerful rifle) or 2) Halo 2 magnum