I coded my own forum about three weeks ago.
We were using myBB on a site, but needed to fully integrate the forums into all else we were doing.
I look at about six or seven forums under the hood and really liked: getvanilla. Elegant code. Made with OOp.
My forums are very, very, very simple, but I am happy with all that I learned making them. (mostly by tearing apart the code of mybb, getvanilla, bbpress, and another I forgot now...)
edit: We also didn't want or need:
avatars
private messages
or all the other bells and whistles
We wanted a basic tool to allow users to discuss issues. Our audience is very, very computer illiterate, with several older folks in their fifties and sixties. Some forums have sooooo much going on that our audience were intimidated.
(I hesitate to post the URL for fear of all you experts hacking it apart, LOL!!! But if you want to see it, let me know...)