The Full Restore item and Max Revive might be the best items in Pokemon that you can buy, considering that they can be brought back into the fight and even turn the tide of a battle, depending on the strategy.
As for held items, things like Leftovers, Focus Sash and Scope Lens are generally the best items for Pokemon to hold. They generally replace HP or status berries about halfway into the game, when your Pokemon can easily withstand most attacks from non-Gym leaders or the Elite 4.
The single best item to me is the Sacred Ash in gen 2, where that would instantly revive all Pokemon in your party one time. Anything that can get you back to the fight, even if is only to waste time/PP and chip off health of an opponent is more than worth the rarity of it.