As a primary melee combatant, Elixir of Spirit Sight (1,000 gp) is a MUST at any middle to higher tier. This item single-handedly prevented a TPK at high tier in a recently released scenario. Gives see invisibility, as well as ghost touch on both your weapons and armor vs. incorporeal. At that moment, was worth 10 times the price.
While very nice, it seems awfully expensive for a one-time use item. It starts to run up against actual items which can give unlimited / daily use of See Invisibility.