Breaking this down.

Garland and Kefka would probably be bottom tier. Assuming Kefka's basic power is shown in the Narshe battle, he's not terribly more powerful than average mooks in other games. Granted his Esper boosted form in Thamasa is probably much higher but I'm going to be stickler and stay with just his "natural" power after infusion. Garland has no real powers so he's probably the weakest. Seymour would also be on this list as he's pretty much a squishy wizard without Anima and even Anima couldn't save him from getting killed. Yu Yevon may also be in this list as well if other Summoners are anything to go by but there is so little info on him it may be safer to just omit him.

In third tier we have Golbez, Zemus, Sephiroth, and Kuja who are frighteningly powerful fighters able to take some of the strongest spells/summons in the series to the face and not be fazed by it. While they are all very powerful they don't exhibit the kind of doomsday size powers of other villains meaning they are certainly not people to trifle with but even they may trout their pants if they had to deal with higher tiers. The Shadowseer is here as well as well as Thamasa Kefka.

In second tier, we have Ultimecia and Emperor Mateus who both exhibit levels of power beyond the rest of the riff raff. Ultimecia's powers are probably no stronger than tier 2 but her unique ability to make thought into reality and give life to inanimate objects may actually mean she can create something that can easily dea with the lower tiers if need be. The fact she's one half of the power needed to even do Time Compression means we may not have actually seen her true normal power before the big battle. Mateus on the other hand is both wickedly smart, can create Cyclones powerful enough to level cities and carry castles, but more importantly he is pretty much a demon summoner who can conjure up an army for the others to deal with and this is before he becomes the Lord of Hell. Not bad if I say so.

In first tier we have Exdeath and Xande, Xande's magic is powerful enough to freeze time and doing so is enough to get the monsters on his side. He's also such a powerful wizard that even Doga and Unei don't seem really inclined to want to fight him head on and they got better powers allegedly. Exdeath on the other hand is an immortal wizard who basically starts with the power to level continents, create energy fields, planet scale earthquakes and fires and this is even before he gets the Void. He's pretty much proves throughout FFV why he is the only one who can control the Void.

God Tier is Cloud of Darkness who frankly could wipe the floor with all of them. Even the ending of III implies she was simply persuaded from annihilating existence this time, but will just keep coming back as she's a part of the cosmos itself. She is basically a god and the equal opposite to the crystals which are the only real omnipotent power in the FF mythos.