I haven't done much experimenting with guitar picks, but I recently tried the Planet Wave's Black Ice picks and fell in love after having used Jazz III's for ages. They are a little bit bigger than a Jazz III and are made of Duralin instead of Nylon. I like the shape, feel and sound of these picks more than any other at current time. They wear a bit faster perhaps, but you can order them in bulk so it's not really an issue.

Here's a picture comparing some of my picks...look at how badly that Black Ice is worn...I play them down to nothing before replacing.

What's are you using? Why?