Miles said:
22x, considering that's the fastest you can burn any DVDs.
It really doesn't matter, but you'll have less chance at a bad burn if you burn at lower speeds.
I suggest 4x or 2x.
The only real reason for burning at lower speeds (2x or 4x) is to prevent coasters caused by buffer underuns. When you are burning a disc, dvd's especially, your biggest problem will come from disk activity. Anything you burn to a disc has to be cached from the hard drive to the dvd drive before it can actually be burnt. When you are on youtube or running a torrent, or using any program that reads/writes to the hard drive frequently, your hard drive wont be able to supply the iso data as fast as the dvd is writing the data, thus you get buffer underruns and coasters. The data that ends up on the disc is identical regardless of burn speed. Crappy media is the biggest cause of drive failures, as inconsistancy in the layer thickness/opacity etc force the laser to constantly re read sectors to get good data. thus causing more wear on the moving parts.
So to sum it all up, make sure you arnt downloading any torrents or running any Hard Drive intense programs and burn at your drive and medias highest compatible speed and have fun.