Amumu vs Sejunai Vs Rammus

Context is king.

Riot is nerfing Rammus and believes he is overpowered, so they will likely nerf him again.

That being said; In terms of direct power overall, Amumu is the best, then Rammus , then Sejuani.... with Rammus possibly dropping below Sejuani after the Rammus nerfs.

In current meta, Raummus. Rammus has some AOE DMG, and good CC and DMG for single person. Amumu has easy to pull off strong CC ult, and medium DMG. Sejuany has tons of AOE DMG, and ult can win teamfights alone, but can be somewhat difficult to pull off perfect ult.

I personally favor Sejuani between the three as a tank. Rammus is stupid powerful against single-threat or AD-heavy teams though.

Currently Rammus is the strongest given the recent changes. His W is awesome. However, if you are running an engage comp, Amumu is a stronger pick. He may edge Rammus out given the teamfight meta in uncoordinated play. Sejuani is certainly better at disengage and counter-engage, but has struggled since the last set of nerfs.