I did my first re-install in 9 months for my windows and now wondering what are the most important and needed codecs are

xvid, ac3 and divx but what else?

You dont need divx, as xvid can play just about any mpeg-4 movie (including every divx ive ever tried).

What else you might need: ogg/ogm, some form of mpeg-2 (from dvd software or whatever). Maybe Quicktime/alt and real/alt?

Thats all i ever do. on the rare occasion i find a file that wont play, i just go find the codec then (tbh, very rare that this happens).

I just stick on VLC as that plays everything I throw at it

VLC and MPlayer Classic for DVDs... QuickTime Alteranitive as well... Thats all I install in the way of Video playing

just for codecs i would say use CCCP, tis the codec pack recommended by pretty much all anime fan-sub groups

supports ogm, mkv etc

will pretty much stand anything you can throw at it