I voted human and elf. I answered strictly from my own perspective, though, not as an estimation of the "D&D genre" or what the "D&D community" might need.
If the PHB had humans, elves, tieflings, dragonborn, and eladrin, I'd be satisfied. I don't view all of those as necessary, but I'd be happy to see them. Humans are humans, and you need those just to cover the generic basis. Elves are the one-with-the-wilderness archetype. Tieflings cover the dark, accursed archetype. Dragonborn cover the proud alien race archetype. And eladrin cover the "more magic than you" archetype.
I could do without dwarves (short, fat, drunk archetype), halflings (... just plain short archetype?), gnomes (...still short archetype?), half elves (weird fantasy genetics archetype?), and half orcs (something initially created to represent the children of bestial rape, but we no longer consider that acceptable, so we're rushing around for a new archetype archetype?). Most of those just aren't different enough from regular humans for me to care about them.
Its possible that I might change my mind on gnomes if they ever develop a strong personality. Dwarves and halflings and half-orcs, though, they had their chance.
If I were going to add something to the list, I'd go for some of the really different races like Deva, Shifter, or Warforged. Races that imply a strong, defining theme.
...although to a certain extent the "races" nomenclature is misleading for some of these. I sometimes feel like insisting that devas, shifters, warforged, and tieflings might be better off if they weren't "races" per se, with cultures and histories and lots of members, but rather as just "things you might be."