Ways that tend to erase evidence but are difficult to execute:
1) Burn the body at 500 degrees.
2) Throw the body in a deep, rushing river.
3) Hack the body into pieces.
Ways that are quick, but can be easily traced:
1) Shoot with a silenced pistol (non-silenced are too loud, IMO).
2) Stab someone while they're sleeping
3) Choke them to death with fishing wire, etc.