I agree that Squalls EX Burst is one of the weakest but his actual EX Mode is the best in my opinion. You do twice as much damage with a character that was already a BP rapist. And for the people saying he can't hit with his HP attacks that just isn't the case, Beat Fang then Aerial Circle hits most characters, And really good Squall players know where the gaps in which to attack are in the enemy defences.
Cloud's is also one of the best, I always forget he becomes unblockable when in EX Mode and always just try to block normally. That goes well.
Gabranth alot of people underestimate because he's so weak when he isn't in EX Mode and people don't like that he takes so long to charge up and become a beast. But with the right accesories you can keep him in EX Mode forever which is deadly over a long period of time.
Firion doesn't have enough good aerial attacks to make him worth using so I don't think his EX Mode wil improve that.
Sephiroth I think has a decent EX Mode, but only because of Heartless Angel which has really helped me out before. But I think Sephiroth criticals enough to do without an EX Mode to be honest.