This one doesn't count but I'm still mentioning it: Necropolis. I can't count it as it's not a supply card, but having this from the start can really propel an engine.
5. Forager - Effective trasher that your opponent can occasionally make even better for you. If it cost more than $3, it wouldn't be worth having, but it costs $3 so it typically is.
4. Squire - After more experience with it, I'm sure now that it's one of the best $2 cards. Gaining attacks for trashing is more effective than you'd think, Squires are easy to pile out, and giving coin is almost as good as giving a card sometimes.
3. Rebuild - This card can seriously warp your objective. Suddenly bum-rushing Duchies doesn't seem like a bad plan. I am still wrapping my head around the card.
2. Knights - Easily the best trashing attack. You know a card is powerful when your opponent generally wants to buy one just to block them.
1. Count - Gaining Duchies for nothing is just sick, and the +$3 is great too. You don't care about gaining Coppers late in the game. If you can get this with a 5/2 start, it can be one of the best trashers in the game because it remains useful at endgame. To top it off, you can tuck away cards for next turn beneficially, making the effect the opposite of a drawback. Count is one of the most handy cards to have around in the entire run of Dominion.
Other Honorable Mentions: Fortress, Scavenger (Double Scavenger BM is surprisingly good), Death Cart, Hermit, Band of Misfits, Junk Dealer