I would recommend all the completes (Mage, Warrior, Champion, Arcane, Divine, Scoundrel, and Adventurer)
and also the races (Stone, Destiny, and Wild)
the guidebooks (Defenders of the Faith, Masters of the Wild, Song and Silence, Sword and Fist, Tome and Blood)
Possibly Unearthed Arcana
The guidebooks take lowest priority, but deserve to be on the list if only because you might need to check your players' character sheets, and possibly to let them look through them for help.
I do not believe in using PHBII or DMGII, i like personalizing my own characters and cities, but if you are stressed for time, the city building can certainly help.
Also, I don't think anything below MM1 is needed, this may be because non of my campaigns hit above 10th level, before we hit a rut and can't meet together for awhile, and when we do no one remembers what happened so we start over. Or, this may be because I design my own monsters sometimes too. Again if you are stressed for time, the MM can be a real help, though do try to avoid randomly picking something from it for each encounter, especially if it's in the same dungeon, i try to vary the circumstances for the encounters inside a dungeon instead of the monsters.
