I'll start:
Adventurer -- The most fun that I have this card is when I'm using it for draw in an engine. Eat your heart out, Venture; you can't do this.
Coppersmith -- Nuff said. If you didn't have fun winning with Coppersmith, I question your humanity.
Tribute -- Rarely do you think "I NEED THIS CARD." But when I can line up the things I want (Scrying Pool, Thief, ORACLE!!!, etc.), I really really enjoy using Tribute. Sometimes I even use it looking for the +$4!
Explorer -- This was the card that I was thinking about when I decided to make this list. I absolutely adore finding uses for Explorer. It gets such a bad rap (deservedly!), but winning with multiple Explorers in your deck is quite satisfying.
Outpost -- Everytime I see this on the board, I spend about 10-20 seconds thinking of some justification for buying it.
Philosopher's Stone -- LOVE this one. I go for this more often than I probably should, it just feels great to buy a Colony with it.
Harvest -- It's just a ton of fun to use, and makes you feel like the success kid meme when you get +$4.
Diadem -- The day when I was first to win a Tournament and I figured out that this was a better prize for my deck at that particular time than any of the other prizes? That was a glorious day!
Stash -- I enjoy the Chancellor-Stash combo, but I also love finding other uses for it. I think it pairs decently with anything that gives you fast cycling, like
a stack of Warehouses or other such things.
[I have no idea of the power levels of DA cards, so none are in my list.]