If asked my personal opinion, I would say that Slackware linux is far better than most other distro's existing, and here's why -
1. Bloat free - unlike most distro's, slackware is completely bloat free, nothing unneccessary is present, and no complicated path is taken to perform simple tasks
2. Control - The level of control slackware gives you over the system is possibly second only to Linux from scratch, its entire functioning is transparent, and system configuration is well laid out in the form of various files, which can be manually edited or edited through the few menu based tools provided.
3. You learn a lot about linux and unix while working on slackware, because every single thing is clearly marked out, so you get to know what actually you are doing.
4. Speed and stability - Slackware is great in both these fields, because of its simplistic and efficient functioning, inexplicable crashes are a rarity.
5. Extendibility - Some features not natively provided by slackware (like a package management with dependencies) are provided by tools like slapt-get, swaret etc.
Those the reasons I like slackware. Another very good distro is Archlinux, it resembles slackware in many features but has some other great things, like an excellent package management. Its a great choice if you have an unlimited internet connection, and like to try out new software and stay up to date.