The best air filter is the one with the most surface area. This usually means a cheap 1 time use paper filter with lots of very dense and very tall pleats. For BMW/Benz/Volvos I stick to Mann or Mahle. Reusable foam filters generally allow way too much dirt through for little or no flow benefit. They try to promote the whole reusable aspect of these filters but when a paper filter costs $7-12 ever 15k-20k miles, do you really want to be driving around with a 100k mile old 'reusable filter'?
http://www.bobistheoilguy.com/airfilter/airtest1.htm