Once upon time... I was MSWin user. I used to encode my video content using DivX codec. The other day I tried XviD. The very next day I removed DivX5 from my system. After XPPro made another suicide, I decided not to reinstall it. Now, sitting behind a Linux box and starting to compile Mplayer, I wonder: On the Mplayer install page the XviD is mentioned as competitor of DivX4. (My experience is it is better than DivX5.x, and faster of course.) And libavcodec is suggested instead.
My question is: is the XviD codec that bad under Linux? What do you recommend for best quality? Can libavcodec really do the best job?

They're all fine.
I use MPlayer/Mencoder and have taken to using lavc because
I'm comfortable with the way you choose options for it with
Mencoder.
I think they're all compliant mpeg4's, and I think most players will
see any of them as "divx".

I don't know about the others, but, lavc will encode mp3 audio.

Well, players read fourcc and if it's set to divx then they guess it's divix. And yes, they all are MPEG-4. But some of them are with better quality, and some are faster. I use ogg audio btw. It's as good as mp3pro and it's free. And ogm container accordingly. ...I think I have to test this famous lavc myself...

The description about the docs is a little outdated. lavc vs. xvid is a hotly debated topic among mencoder users. IMO nothing comes close to xvid's quality, and it's performance under linux is equally good as under windows. There is a good comparison of the windows version of these codecs by doom9 available here. 'ffvfw' used in this test is a windows implementation of lavc. Although the test is a bit dated, neither lavc or xvid has changed drastically since then.

Thanks, this is the answer I asked for!
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XviD it is.