galvanised metal - cheap,used by loads,yes, the odd sharp edge but not that much of a problem won't sag on bottom bee space if put directly onto frames, but then has to be peeled off, no big deal but not for me, can't be scorched as the thin metal will distort
Same for plastic (can't scorch either but for different reasons!)
You can put in a frame to give bee space on both of the above, but you neeed a central rib to stop sagging.
Framed wire - robust, no sharp edges,rather nice rounded wire. Can be lightly scorched, has got bee space but a lot more expensive.
Framed wire for me every time though.