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.

Dexter, you're a boy, make a big noise

Playing in the street, gonna be a big man someday

You got Molly in your face, you big disgrace

Discarding assets all over the place, singin'

We will, five Will shrivel you

We will, five Will shrivel you

Drake, you're a young man, soldier man

Fine clothes in the street, gonna take on the world someday

You got Molly in your face, you big disgrace

Waving your haste all over the place

Six Will, we will summon you, sing it!

Six Will, we will summon you, yeah

Great Drake, you're an old man, veteran

Pleading with Yaztaroth, gonna get you some peace someday

You got Molly in your face, big disgrace

Summoned hound put you back in your place, do it!

Seven Will, we will razor you, yeah, yeah, come on

Eight Will, we will wither you, alright, louder!

Nine Will we will shrivel V you, one more time

Ten Will, we will shrivel you

Yeah