Prosperity is my first expansion, and it is the one I chose because it sort of gives you dominion but larger.
I found that sometimes we would play dominion as a 4 and the game wouldn't take long at all, I wanted something that extended it.
Whilst on holiday we played this about 4 times and 1 game was almost excruciatingly long, it took ages, but the other three were much better, one was quite short but the other 2 were of a decent length. The new cards (which we only played with, no base game cards - except the coins and property) did have some interesting effects and we used most of them, the game were interesting for us 4 as we had all played dominion basic plenty of times. I think if you'd like Dominion to be a bit longer or bigger then this is a definite expansion to try. I looked into all the expansions and this came out on top for me, maybe intrigue next, but I'd like to see how it works if we have a true mix of both base and expansion cards, might end up a bit hotch-potch.
Dead of winter was great, everyone but me won, they all had easy targets like having a few medicine in hand, I had to ensure there were loads of barriers at the basecamp and they all got destroyed and overrun by zombies on the last go, but as I said it was late and only one other player was really enjoying it, so it might not come out for a while. I thought it was good and a nice long in depth game of it would be great to play, hopefully I can convince my wife to play it with me - once we get Avalon to sleep a decent amount of time.