Which single item offers the highest amount of cooldown reduction?
As of today, 3 items give the most cooldown reduction, which is 20%:
65 Ability Power, 50 Attack Speed, 10 Mana Regeneration. UNIQUE Passive: +20% Cooldown Reduction.
75 Ability Power, +20% Cooldown Reduction, 12 Mana Regeneration. UNIQUE Passive: Dealing magic damage to an enemy champion below 40% health inflicts Grievous Wounds on them for 4 seconds. (Grievous wounds reduces healing and regeneration rates by 50%)
90 Armor, +20% Cooldown Reduction, 400 Mana. UNIQUE Passive: Reduces the attack speed of nearby enemies by 20% (1000 range).