Having hard cc is more important than any other aspect when picking a team comp.

You could be 5 of the highest damaging champions, or whatever, but if you are pretty shy in the hard cc department, then you will likely struggle. Who needs tons of amazing damage when the enemy cant attack you anyways. Let your team have those 2-3 carries or so, mix in some hard CC (AOE preferable), and you got a good team comp for most situations. Also, hard cc that has a delayed application, for example Fizz's ult, is less preferable than the instant proc kind like a Alistar Q or a Galio ult, because you give the enemy a longer window to react.