I think whips are supposed to be the most potentially dangerous, with crops and floggers maybe in the middle? Whips are something that should only be handled by experienced players. I hear of one couple who met online and tried a whip on the first day they met (had only experience in cybering), and the guy sliced the girl's leg open with the whip.

I've seen some pretty nasty marks left by canes and some icky bruises from crops, but I'm not sure how easily those would break skin. Floggers could probably make some nasty marks depending on the material and how the ends are finished (and how they're handled). Paddles I would say look to be the least likely to do more than bruise and pop blood vessels under the skin (leaving hickey-like marks). Unless someone spiked the paddle like they do with vampire gloves...

Just speculation here:
Paddles, canes, and crops sound the most straightforward to use to me. Those seem like the depend on just the build/material and how hard you swing (and, of course, where you hit). Whips and floggers I've heard described as more of an art, probably because of the flexibility? Like, you can make a whip snake up where paddles have to go flat. That means more can affect the strike strength I would think. less