For me the most underwhelming rune is overheal. At best it grants around 200ish shield when fully stacked at lvl 18 on most squishes, but lasts only 6 seconds before decaying rapidly meaning the shield really doesn't stay long. It just feels rather useless compared to other runes to it. Triumph alone makes it feel basically a useless rune. When the next rune over does basically the same thing give more sustain/health to survive a fight, but gives more health and gold. Even Presence of Mind is useful as it allows for unlimited mana for 7 seconds on lvl ups or kills, making it extremely useful for champs that have low cooldown high mana cost abilities. There is just no situation where overheal is a good rune to use over Triumph.
What rune do you find most underwhelming?