Amumu

Strengths

Amumu is highly dependent on teammates, so try laning with your friends for maximum effectiveness.
Cooldown Reduction on Amumu is very strong, but it's often difficult to itemize for it. Grab the Golem buff whenever possible to gain Cooldown Reduction without sacrificing stats.
Despair is very effective against other tanks, so make sure you're in range of opponents with the highest Health.

Passive · Cursed Touch — Amumu's basic attacks Curse his enemies, causing them to take bonus true damage from incoming magic damage.
Q · Bandage Toss — Amumu tosses a sticky bandage at a target, stunning and damaging the target while he pulls himself to them.
W · Despair — Overcome by anguish, nearby enemies lose a percentage of their maximum Health each second and have their Curses refreshed.
E · Tantrum — Permanently reduces the physical damage Amumu would take. Amumu can unleash his rage, dealing damage to surrounding enemies. Each time Amumu is hit, the cooldown on Tantrum is reduced.
Ultimate · Curse of the Sad Mummy — Amumu entangles surrounding enemy units in bandages, applying his Curse, damaging and stunning them.

A suggested 5-lane lineup built around Amumu, based on hero partnerships and role synergy — not win rate.