Top 5 on 5: Best Pen and Paper RPG Systems

This is Nihzlet, bringing back a monthly article I planned as part of my original blog, Retro Strikes Back. The basic idea is simple--on the 5th of the month, I'll post a personal top 5 list of some kind. This month, since it's the first time I'm posting in a blog that's not just about video games, I'd like to take a moment to talk about my preferences in Pen and Paper RPGs. As a general note, since the roleplaying in any game is really up to the players, not the system, I'll mostly be talking about mechanics and adaptability of the system. Without further ado, let's get started!