Rats is already way too high, it's a deck-destroying trap card on more than 9 out of 10 boards. Should definitely be bottom tier. I voted it second-to-last myself, just above Scout.
I don't think how good a card is when you randomly add it to a deck is a good measure of the goodness of a card. You buy Rats when you shouldn't and you definitely play the card from your hand too often. Doesn't make Rats worse than Thief and Spy. Rats has its place, Thief just really doesn't.
Every card has its place. Thief is good in some games with strong trashing but no virtual money. I think Rats is more situational than Thief, it is generally weaker and shines on a smaller amount of boards. I don't think I've actually bought Rats more than a couple of times in my life, and I don't think I've EVER been glad about buying it, nor have I regretted not buying it (then again, of course I didn't play Dark Ages half as much as Base...).
Before people come out and show me games where they dominated with Rats: I know they exist, I just acknowledged every card has its place, I'm just saying that Rats is worse than almost all other cards in general.
I agree Miser doesn't belong down here, it's really awesome if you can play it often. I Princed a Miser once which is absolutely crazy. It was basically +$6 every turn at some point.
Okay, but you really don't get to Prince it every game. Cards being crazy good with Prince doesn't make them like, ever good in general... you could say the exact same thing about Pirate Ship and there's a pretty clear consensus on that card.
But Pirate Ship actually helps your opponents a lot of the time, while Miser clears out your own copper. Of course, Moneylender and Spice Merchant do it better, as they provide you immediate benefit, but imo Miser is clearly better than Pirate Ship. Miser is not just good with Prince, it also works nicely with Throne Room variants and strong engines in general. If nothing else, it's an acceptable pseudo-trasher, if you want to build an engine but lack actual trashing.