I still need to get the D series (I have Against the Giants). I'm skipping Queen of the Demonweb Pits. I also need Principalities of Glantri, and the first Dark Sun boxed set.
The last big holes I filled in my collection were the Saltmarsh series, Sabre River and Death's Ride for the Companion levels, the Planescape boxed set (for $15, and it came with 2 adventures) and the D&D dm's screen.
My collection is mostly in storage boxes, but I drag them out occasionally to look at them.
Update: I just ordered D1-2 Descent into the Depths of the Earth, and D3 Vault of the Drow.