I think I like "tied up" and "candles,clamps and plugs" the best.
I like to picture myself in the place of the helpless model in the picture. Also I'm a big fan of anything fashion so it also adds to my pleasure if the model is dressed in tasty clothes. I also like the "c,c&p" variety because those usually have the "captured model is now turned into a toy" feel to them.

PS. Also got to love breast torture.