Capital + Storyteller
Play a storyteller with capital for 6 extra cards. By itself, this isn't anything special since it still gives you debt, but if you can find a way around the debt via a megaturn or just not buying things, it can be powerful. To illustrate, with artificer you can draw your deck with storyteller+capital and use artificer to gain more of each of these cards. Once you get going, you can just start gaining provinces with artificer and drawing back the cards you discarded with storyteller+capital. I realize that Storyteller + Artificer is itself a strong combo, but Capital really seems to do a lot with it, keeping you from having to buy more than a couple other treasures.
I mostly just like the idea of accruing a huge pile of debt that you never intend to pay back.