The order I would place things is;
A sheet metal sheer is going to give the straightest, cleanest edge.
A nibbler (at least the ones I've used) cut tiny round bites out of the metal so you can go in any direction (round corners, etc.)
A nibbler is as straight as you make it go and leaves a somewhat serrated edge.
A plasma arc cutter can cut in any direction and is as straight as you can make it. You can use a straight edge of some sort to help the cut be straight. The edge is burned but can be pretty nice.
A cutting torch will leave a burned edge and will take some practice to leave nice edges.
Plasma arc (to me) seems my choice for space and versatility.
