Seconded!As much as I don't especially care for the games he came out with Mark Rein*Hagen - the dude created work that challenged (and maybe for a month or two outsold) D&D and defined a decade with his book's approach and playstyle. Personally I don't think that Planescape would have existed, or at the very least, looked very very different without the Storyteller games.
I actually care about what he created, though. Ars Magica is high on my list of rpg systems that I consider to be the best ever created.
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