Estates.
Copper does NOT hurt poorhouse until you're playing multiples at a time.
(If you play PH with no coppers in hand, you get 4 coins from it. If you play ph with one copper in hand, you get 3 coins from the ph and one from the copper for a total of four. And so on and so forth - if you're playing a single ph in a turn, you get $4 regardless of the number of coppers in your hand (as long as it's below 5)).
This seems to make thematic sense - the Poor House gives to the poor - it evens out the social inequality to make sure you have at least $4. So if you have nothing, the Poor House gives you $4. If you have two Coppers, well, the Poor House gives you $2. If you've got five Coppers, well, you definitely don't need anything, and for good measure, we're taking the revenue you got from your Market. So there.