
I think Dexter is the strongest Mystic investigator yet. His power gives +2 economy (including saving an action, the most precious commodity in AH) every time it's used, and it isn't too hard to set it up. He makes cards that are already strong even stronger (e.g. Arcane Initiate, David Renfield and Scroll of Prophecies, as well as the typical suite of spell assets), brings decent cards into the strong contender range (e.g. Forbidden Knowledge), and even makes a few obscure cards worth playing (like Alchemical Transmutation). The reason his power is so strong is the same reason why fast effects and cards are so good: saving an action and breaking the normal restrictions on play windows can be incredibly useful, especially on harder difficulties.