I don't think you will get one and the same answer from users. It may also be a matter of personal preference. I personally still use DivX5.2.1-codec (also to decode XviD encoded movies) though I've also installed XviD-codec (for test purposes). Further I've also installed ffdshow-filter on slower systems. Users like
adicoto and
Tizio will probably advise for either or both codecs, whereas
J7N will, no doubt, advise ffdshow-filter.
P.S.: The most important thing is that it works! (and that you don't install to many codecs!)