1. Cafe World
Great graphics, lots of cool themes with great art! The themes (and decorations) which are introduced every few weeks helps to keep players hooked on redesigning their cafe. Helps to fund Zynga's development as people will spend cash to buy those rare, desirable decorations.
Lots of people keep fish as a hobby. Developing this into a game is common sense. But the idea of buying rare fish with real cash and then selling them is a bit extreme I think. Good business sense for Zynga though.
Didn't initially get hooked onto this game, but now I'm a fan. Graphics are not as good as Cafe World's, but there's full-screen mode which is a huge bonus to enjoyment of the game. Many people have gardening as a hobby. Planting and growing crops will appeal to this group of people.
4. Mafia Wars
This is one of the few Zynga games with no Flash animation. But the gameplay is fun and intuitive. Interesting concept too.