My preference is panty upskirts. Nude upskirts don't really interest me. But I understand it interests others.
As @bugzme says, there are lots of sublte nuances with upskirts.
I like the subtle ones where the model has her legs closed and you only see a triagle of material. This is more voyeristic.
I also like the ones where the model is sitting at an angle with her legs a little open. Again you see a little bit of material
On the other hand, I also like where the model opens her legs quite wide for a brief moment. In video, you get good upskirts when the model shifts position. Some of the best upskirt moments are often like this when the you get a glimpse and it is covered again, followed by another glimpse as the model shifts position.
Then there are the more deliberate updkirts. I also equally like the full frontal views and panty flashing.