What gem benefits the most from quality?

empower and enhance are strong #1s

after that, probably flameblast (30% chance to ignite - frees up 'chance to ignite' slot), blood rage if low life (~96% attack speed) and some curses (mainly ele weak, temp chains and enfeeble)

Now that it's a viable build-around gem, I'd say Detonate Dead. 60% increased cast speed is insane, that's about as much as one could expect by building around cast speed and this skill gets it for free with 20q.

Freezing Pulse is a useless gem without 20% quality so I'd say that.

EK or Freezing Pulse.

Freezing Pulse
Flameblast
Incinerate
Lightning Warp
Shock Nova
Detonate Dead
Blood Rage
Cyclone
Explosive Arrow
Frenzy
Temporal Chains
Immortal Call
Infernal Blow
Melee Splash
Added Lightning Damage
Increased Critical Damage
Minion and Totem Elemental Resistance
Power Charge on Critical

All of these get a pretty big part of their effectiveness from quality.

For a whole 'subset' I would say curses get the most bang for the buck from quality.

Then a few niche ones like someone mentioned Zombies. 20% health is nice, but nowhere near the improvements that curses get. They nearly double in their power.