Oh, just get one of everything! If you really want a nutty variety, get a pekin, a muscovy, a runner, a welsh harlequin and a wood duck... they represent the extremes of duck breeds to me.
Actually, You could start out with all runners-they are hysterical to watch run around, and come in all different colors (they are available in a zillion types!) or all muscovys...also available in many colors... If you want super friendly...get less than 5 day old ducklings and hold them every day and give them a million treats... that generally does the trick.
Good luck! You'll be a duck addict soon!
My favorite duck for looks has got to be a silver appleyard... but they fly and are a little harder to come by. My dad thinks wood ducks are the best lookers.