I think it depends upon the class.
Rogues can be pretty close to optimal in the purest sense of your analysis here: no multiclassing, skill training, armor, or weapon feats.
Archer rangers benefit a lot from getting greatbow proficiency but can otherwise do fine without multiclassing or more armor.
TWF rangers can be close to optimal with twin scimitars and the stormwarden paragon path but most other TWF rangers will probably want to take a superior weapon proficiency. A lot will also multiclass for paragon paths (usually from fighter, but occasionally from cleric); most of those will also take armor proficiencies.
Paladins can do allright without multiclassing. Charisma paladins can be acceptable without weapon proficiency feats, but of course are better once they take them. Strength paladins really need to upgrade to a superior weapon and have to multiclass if they want to have a level 9 daily power they can hit with.
Laser clerics do fine pure. Battle clerics want a superior weapon and superior armor proficiencies and probably want to multiclass to have a decent paragon path. (This is pretty much necessary if they don't have a 16+ starting wisdom).
Fighters want superior weapon proficiency. After that, they can do ok pure, but most will multiclass a little.
Wizards are ok pure, but most will take an armor proficiency.
Warlocks (other than fey pact) will usually multiclass in order to have good paragon path options. (This may be different now that arcane power is out; I haven't looked at it too closely).
Warlords will generally want a superior weapon. Charisma based warlords will also generally want to improve their armor though some won't have the con to be able to do so. Warlord paragon paths and powers are quite good however, so most will not multiclass (or will do so primarily for the multiclass feat itself).
Of the PHB 2 classes, nearly all avengers and barbarians will take superior weapon proficiency and most avengers and most thaneborn barbarians will take armor proficiencies too. (As will many rageblood barbarians). Most barbarians and most avengers will also multiclass for improved paragon path options. Druids look to be ok pure. So do shamans. Wardens will usually pick up a superior weapon but they're ok after doing so. I think bards want a superior weapon and probably want to upgrade their armor, but I don't think they need to multiclass in order to be effective--at least valor bards have the absolutely amazing warchanter paragon path and most of their powers are decent. Invokers look ok absolutely pure.
Sorcerers will usually want armor proficiency. Multiclassing and weapon proficiencies seem pretty optional though their stats open up a lot of cherrypicking options for multiclassing since they share primary and secondary stats with a lot of classes (warlock, warlord, paladin, rogue, fighter, warden, ranger, bard, etc).