Each of us will have a different answer! It really depends on what you make the most. I make mostly APAs, IPAs, and a few steam beers.
So I bought amarillo, northern brewer (for my steam beer), cascade, magnum (for bittering) and willamette. I have no idea why I bought the willamette, as I never use it, but it was $9.25/pound and I thought I'd give it a try.
If you make more English styles, you'll want fuggles or East Kent Goldings. If you're a Belgian fan, you'll want Styrian Goldings. For mostly German beers, you'll want hallertauer or tettnang. If you're a pilsner nut, you'll want saaz.
Generally, for American style beers, a couple of "c" hops: centennial, cascade, chinook- are what you'll need. I have grown to love amarillo hops in my IPA, so I use that often. I also use tons of cascades.