It all depends on the styles of beers that you like. If you want to do a Two Hearted, the Centennial is a must have. I like to keep Biermunchers Centennial Blonde on tap so that also includes cascade. The cascade is also used in EdWort's Haus Pale Ale.
For Hopsdirect price of $6.75 and it's universal use I also keep Willamette on hand. With the current prices, it sure makes it tough to decide, but it is nice to be able to buy 2 or 3 for the same price I've paid for one recently. Good luck deciding.
FERMENTING: BCS Black Forest Stout, Two Hearted Ale Clone
ON TAP: Dry Irish Stout, Epic Pale Ale Clone, BM's Cent Blonde, Blackfoot River IPA, Racer 5 IPA clone with homegrown hops, Oatmeal Chocolate Stout, Threee Floyd's Gumball head clone, Moose Drools Clone
ON DECK: Saison, Pliny the Elder clone