The best most simple and stable media player would be VLC player. It plays a large varity of different formats, pretty much every player under the sun plays .avi because it is the most widely used format, excluding rare encoded avi's. VLC is perfect for those rare formats and also formats such as MKV and OGG which can contain multiple audio streams. It is also one of the best for the newer H264 encoding as well as Xvid (but pretty much every media player under the sun plays that also). It also plays DVD's
You can download VLC player from here:
VLC Media PlayerMy favorite I use is K-lite Mega codec packs Windows Media classic. I use it most of the time because it plays all the same formats as VLC can and also it displays Hardsub subtitles much more clearly then VLC is capable of. The pack contains lots of different codecs along with Divx software so you can convert formats to whatever. This windows media player classic and the combined codecs within this pack will probably plays more formats then VLC.
You can download here:
K-lite Mega Codec PackI would reconmend you download both, so if you manage to find some video one program cant play you can use the other as back up.