Dominion expansions

We've played Dominion pretty heavily since purchasing it and I think it's time to start adding expansions. Which ones are your favorites? Why? Any reasons to purchase them in a certain order?

I think I'm leaning toward Prosperity or Seaside, but part of me thinks I should just get them in order and grab Intrigue next. It would occasionally be nice to play with 5, but our typical games are 3 people.