Which mage is the most useful in a team fight out of the following...

isn't a mage isn't a mage

There's no answer without knowing the rest of the comp, what the enemy team comp is, how the champions are doing in game, what point of the game you're at, etc. League is far too complicated of a game to simply say x champion is better than y.

Cass brings good aoe cc, a grounding effect, and very very high sustained damage Morde brings inherent tankiness, the ability to take one champion out of the fight entirely, and a pull to ruin positioning Cho brings a good aoe knockup, a high health execute, and a silence

All of those tools can be extremely useful, or redundant depending on dozens of other circumstances.

I'm divided between morde and cho'gath.

But I'm biased as an ADC. I like front line champs better.

morde is overall better since he can take someone out of the fight, just needs a second frontliner on his team

Cho.

with his natural tankyness combined with reasonable positioning will make his CC devastating.

alternatively he can pop and DEVOUR for a 'nice counterplay' moment in dealing over 1800 true damage and deleting the enemy carry.

on top of this he is allowed to build either AP and 2-shot people, or build tank and go for the aforementioned 'nice counterplay'. giving him more versatility that the other 2, combined with potentially near-infinite scaling HP and teamfights should be easy.

(on that note QSS, zhonyas and other CC-removing abilities counter both morde and cassiopia, while cho's knockup cannot be cleansed.)

That's all.

Team fights are about both teams and all members of them. If there's a lot of dashes on the other team, Cass may be best. If your team has 1 tank AND they have no tank-buster, Cho. Else, Morde.

Its situational, If your team is full of squishy champs and you need a frontline: , If youre fighting around baron and want to take the enemy jg out of the equasion to secure, or want to remove that fed adc from the fight: , and if your team already has frontline but needs extra damage:

Cassio is good at stopping engages with W Cho has a silence and is a tanky boi Morde gets to pick 1 person in particular that he doesn't want his team to have to deal with and take them out of a teamfight

Cass since she is the only mage present she wins by default

All Mages are useful in their own way.