You might recall that SilverStripe was last years #1 most promising cms - well let's help see that promise fulfilled :D
It's interesting to see MODx and XOOPs up there - I've heard a lot of talk about these having a similar sort of developer experience, so it's great to see them on the list. As for Wordpress, well, it's a mess, but it's a friendly mess with a lot of followers and DotNetNuke is really all the ASP.NET guys have got :P
Having developed for wordpress quite a bit, I have to question its status as "best" open source CMS. Its pretty easy to use, and has loads of plugins, but it's not particularly intuitive if you have a site with over 10 or so pages.