Assuming we're talking about locking folders, I'd say one of each common lock type. Of the major ones that I would consider :
Ball bearing (Dodo, Manix2, Poliwog)
Lockback (Dragonfly, Delica, Calypso)
Liner lock (Military, Bradley, Sage)
Frame lock (Southard, Domino, Tuff)
Compression lock (Paramilitary2, Yojimbo2, Junior)
I'd probably try to get a good variety of blade steels and handle materials between them as well.