I'm using mambo. I Think it's the best choice.

there are more cms then that.
there is MKportal http://www.mkportal.it/
that is quite good, i tryed him, you just install him over your fourm and plus you dont need another mySQL databes becouse it uses one of the forum.
Mambo is good, but now its joomla becouse mambo started paying for services or something like that.
then there is typo3 that is very complex...
for php-nuke... no comment, use php nuke only if you wont some wargame for hackers on your site
Xoops i didnt try but here you are a link to try demo of a lot of cms portals:
http://demo.opensourcecms.com/
or to compere cms's:
http://www.cmsmatrix.org/matrix

hm i dont know which i like best. i think i would say xoops because its easy to use. from i like mambo a lot to. for me mambo is just very difficult. and its hard to get out of that mambo look and to make your own template. i dont know if this counts as a cms but the blogs the are from blogspot.com are very cool. the templateing is like the easyest thing ever.

I've been using PHPNuke and Mambo and choose Mambo between those two. I don't know why, tough. May be because of easiness of creating templates for it, and a lot of translations, modules, and alike stuff out there.