#RPGaDay2016, Day 20 - Most challenging, but rewarding system you have learned?

The system I have used by a far far vast majority is the d20 System. In retrospect, it is a pretty crunchy system. But with the sheer amount of playing of that system, it has been the absolute most rewarding.

For all of it's flaws, I am still quite fond of the system. Particularly, Spycraft and d20 Modern.

We still play it today with no end in sight, as my group won't give up Pathfinder.