Unlike some other cards on our Top 5 list, Stoneforge Mystic is played in a multitude of decks ranging from Hammertime to Azorius Control and he is brilliant everywhere
Gimmicky planeswalkers are ones that have a twist to them, only having negative abilities, maybe being an uncommon walker, a nonlegendary one, one centered around only zeroes, one that transforms, one that has a plus that can get it to an ult directly by adding loyalty counters, etc
Once on the battlefield, it refills your hand with cards equal to half your artifact count, trading life for resources
My list doesn't focus on cEDH at all, but I can imagine this has a home there, given the sheer amount of stack interaction in that format