A plasma cutter is the fastest method, however not many of us have one. I prefer using a cut-off wheel (12,000 rpm rating) in my air grinder or an 18v DeWALT angle grinder. If I am rough cutting, I will use a reciprocating saw with a good quality metal blade. I have also used shears and nibblers. Each of these tools have their place and are effective, however it is a matter of preference.
Tony
