There is no best - depends on the person and the purpose. For me, it's Slack by light years, but that doesn't mean someone else can't hate it.
For security, there are several specially hardened distros you can find at distrowatch and like places. As far as standard distros, I believe Debian and Slackware (really) have the reputations for being most secure though I haven't exhaustively analyzed any data to the effect so if you're a certain kind of person, feel free to disregard that hearsay.
-- Ah. Just spotted the post count. Welcome to LQ!