I'd definitely have to say Gentoo, because of its Portage system. And while Portage can sometimes have its problems, it's largely been very good to me, and has allowed me to keep very fine-grained control over my system with minimal issues. It's also really damn easy to tinker with your entire system (all forms of upgrades or rollbacks are usually painless), but very difficult to get your system into a bad state, where extensive troubleshooting would be required (as the more control you have, the easier it is to mess things up).
But other than Gentoo, no other Linux distros really tickle my fancy except for Arch Linux (and possibly Slackware), as the only types of distros I use anymore are "meta distros", which are essentially distros that let you build your software from the ground-up.
It shall also be noted that, for the record, I'm new to FreeBSD, and have only been using it for a couple of days. Admittedly, the main reason for doing so is because I heard that Portage had a lot in common with Ports - but also because a *BSD operating system has been something I've wanted to try for a while now.
So far, I like it, but I'm going to withhold my final judgment until I get some more experience with it.