Hehe.. I was afraid that this thread will decend into the "mine is bigger than yours" syndrome, but that's expected I guess.
From my point of view, I would download, install, test and use all of them. Then choose whatever suites you best. Heck, they're all free!
There are arguments for/against on every product. I personally prefer PostNuke for the reasons I stated previously and it's ability for infinite configurability and customization which makes it ideally suited for a lot of applications.
For instance, I have PostNuke installed in my office as a "knowledgebase" center where all documents, articles, prices, etc are in one place for all the users to get instantly. I didn't need several modules like the downloads, avantgo, and other bits and pieces, just a straightforward install without any bells and whistles.
For Bahraini.tv I've installed the necessary modules to facilitate a "community" website.
Again from my personal experience as far as security is concerned, I have found the PostNuke team very responsive to any security holes that are discovered, and if you read the developers' lists, you will find that they actually unselfishly propegate that information to other *nuke based engines including PHPNuke to bring it to those developers attention.
Now as for the future, the published roadplan is really fantastic and has everything I need and am happy to grow with it. I don't have to install every upgrade that is released, but I find that the upgrades either add functionalities, squash bugs discovered in the interim or patch whatever security holes that have been discovered.
No CMS system can claim to be the be-all and end-all product, not even ones costing several thousands of dollars... however, I find PostNuke to be well worth having and is a joy to operate.