Wordpress, SMF, a little Joomla. Was using phpBB, but not using it now as it is pathetic due to spam issues.
Wordpress is the best for content management. If you want also services you should use PHP Nuke, Drupal or Joomla
I also use vBulletin as the basis for 1 of my websites, as a CMS substitute. Maybe it's just because I'm used to it, but I love the power and flexibility of vBulletin! I also use pligg, and wordpress (both normal and MU versions). Surprisingly (to me, at least) I've never used drupal, even though it has been recommended it to me countless times!
Currently i'm just about to start using WordPress as i have seen a few other sites and it seems to be the best tool for site backends these days Before that i used Joomla and i never really liked it
Just started with WordPress, and I'm impressed. My anniversary gift site in the sig is with Joomla. Joomla was a bit confusing to me at first, but then again, I was new to the whole CMS thing. I must say, it's fantastic, and the SEO modules and active developer community are simply amazing! For smaller sites, I would recommend WordPress, but for a larger site, Joomla is the way to go, in my opinion. Paul