We need upgrades pretty much from every position except point guard. But if I could only pick 2, I'd take upgrades at the 5 and the 4, in that order. Or 5 and 3. This is assuming we get these upgrades without giving up any of our current top players (say, through free agency or the draft).
Obviously it would be great to draft Simmons but I'm not holding my breath for that to happen.