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I've worked with PHPNuke, PostNuke, Drupal, phpWebsite, phpBB, IPB, vB Board, Burning Board, phpGedView, Movable Type, Blogger.... just to name a few. I've worked other, more obscure, portals, Blogs and forums as well, but I can't recall all I've worked with in the past two years. Some, I gave up on from the first day, some I've actually put into production.
As a whole though, I feel XOOPS has offered the best security, feature set, and stability combination of everything I've worked with.
At this point in time, I'm centralizing my efforts on learning the ins and outs of XOOPS as well as learning PHP and MySQL so that I can offer more to the XOOPS community and my clientel.
I personally see no sense in looking any further the the "Best" CMS. I've already found it. Now it's just a matter of developing my skills so that I can contribute to making the best CMS better.
As far as learning PHP compaired to other languages goes, Apples and oranges.
I can't speak on C++ or Java, but I can say that learning PHPis a sharper curve than HTML and CSS. Regardless, the benefits of the knowledge you gain will far out weigh any effort required.