Bookkeeper
Action - $3
+2 Actions
+1 Buy
You may discard any number of cards for +$1 each
Setup: Place 5 copies of Bookkeeper on top of 5 copies of Counting House
Concept: Bookkeeper ensures a few of the things that make Counting House work are in the kingdom with it, and it provides a nice synergy, but both it and Counting House combo better with other cards
I, as the only person I know of to date who has Counting House as their favourite card, I wholeheartedly support this card. As for my entry, let's try to improve one of the worst cards in all of Dominion.
Path/ScoutPath$3
Action-Victory
+1 Action
+$1
Gain a Path. If you do, +$1.
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0VP
ScoutYou know what Scout does.
5 copies of Path on top of 5 copies of Scout. Guarantees an Action-Victory for Scout to work with, and synergizes with Scout. Path gains copies of itself and runs out its pile so that Scout can be uncovered most games. Hopefully this improves Scout enough for you to actually want one.
Edit: Made Path give +$1 instead of +1 Card, making it more useful as payload for Scout to draw. Hopefully that improves it in a similar way that choosing +1 Action +$1 is often better than choosing +1 Card +1 Action.