Pirate Ship. I realize I'm probably biased, and I also realize that it's not the best card ever, but to me, it seems very well designed. It does exactly what you'd think a Pirate Ship would do. I always imagine myself looting and pillaging whenever I let loose a Pirate Ship. And the art is pretty damn good, too.
As for the expansion fit, Pirate Ship is not a card that is good for you right away. In fact, it's almost bad - you're helping your opponent get rid of chaff in their deck. But if you use it well, Pirate Ship is a juggernaut in the late game - it can yield more coins than any other card of its cost, or really, any other card in the game, aside from perhaps Vault. This ties in quite nicely with the "next turn" theme of Seaside.
Honorable Mentions:
Young Witch - BEAUTIFUL artwork, probably the best.
Harvest - it really feels like you reap what you sow with this card.