What's the most rarely used rune?

futures’ market. Double edged sword rune if i have ever seen one.

Unsealed spellbook and that new 3 AA domination keystone are probably the rarest keystones Future market is probably the rarest rune overall

I don't think I've seen a single person use spellbook since they reworked conqueror and Riven and Urgot started using that instead.

font of life.

Do many champs use Guardian or whatever the 3rd resolve rune is, I only play ARAM but I see this rune so rarely and it doesn't seem like would be much better on the rift.

If not guardian unsealed spellbook is probably least used keystone ingame.

If we are talking keystones, then probebly usealed spellbook. If we talk all runes then the one i never use is approach velocity.

I use Future's Market all the time on ARAM, since passive gold gen is higher there. Makes it easier to get out of debt.

I'd say Font of Life is the least-used rune overall. The healing it gives is so pathetically low that even enchanter supports are better off taking Shield Bash (in the case of Lulu/Janna/etc.) or Demolish (Soraka, since she tends to get a lot of HP items).

Nullifying Orb and Celerity are pretty rare as well I think. Orb's shield is just too small to be useful, and Celerity was nerfed several times to where it's barely noticeable even on the champions it was designed for. Unflinching is also something I never see, probably because Overgrowth is more reliable for tanks.

I actually think Hextech Flashtraption is my least used rune. It's a good rune, but I rarely find myself in a situation where it's beneficial to channel a flash as a gap-closer or to cross terrain. That is probably because most of the champions I play have abilities that can do that.