I can't really tell you much about the best format, I hardly ever d/l movies (those that I have d/led were .avi)! Use the search (top right) & look for best format, movie download format or something similar.

Before you do d/l a file, check it out with Bitzi. If the file's been rated, you'll get an idea of content & quality. Saves wasting time downloading misnamed porn
How to use Bitzi

Your download speed depends on lots of factors - your connection, host's connection,number of hosts you're downloading from, whether you are firewalled, how well you've got your LW set up, whether your ISP uses traffic shaping....on it goes. Make sure your LW is optimised
Configuring LW for best performance
Try to download from multiple sources with a 4 or 3 star connection. Preview your download after about 15% to make sure it looks ok.