I have to say…. Mages. The rules for Mage: the Ascension (as I recall them, I don’t own the book) seemed to be as far from anything like traditional magic (or even D&D magic) as one could get. The intention as I understand it was to get away from the limited spellcasting rules of D&D type games and give players some real freedom and creativity with casting spells, which to be fair, it did. But magic is always easier to create consistently in fiction than in a game. Magic is hard to quantify, reduce to mathematical formulas, statistics and clearly defined levels of skill and accomplishment. Within the context of a Mage chronicle, it probably worked fine. But it made them almost impossible to use in crossover—mages always seemed way overpowered compared to vampires, and the whole cosmology didn’t seem to match up.
I know there are a lot of Mage fans out there, but it just never clicked with me at all.
I did like the Mage: the Awakening system – but I’d still hate to try using it in crossover. (Though it was fun to let Regina fry Loki’s ass when he tried to ambush her in Three Shades of Night. Stupid punk vampire. Shoulda known better…..)
But anyway. If I have a least favorite, the Mages would be it.