Let's discuss about each one.
All of them have a titanic movepool with very useful moves.
Blissey and Clefable have Softboiled while Wigglytuff only has Wish.
Blissey has atrocious Def though monstrous HP and Sp.Def.
Clefable has very high HP, decent Def and very resepctable Sp.Def.
Wigglytuff has barely passable defenses and stellar HP.
Although Blissey has the less Def, they all take approximately the same damage from physical attacks.
Here is a calc I did.
Salamence Adamant 252 Att Dragon Claw vs 252 HP 252 Def Bold : Blissey - 37.5% - 44%
Wigglytuff - 35% - 41%
Clefable - 33% - 39%
As you can see, Blissey only takes 3% more than Wigglytuff and 5% than Clefable. Not too much more.
On the other hand --->
Starmie Timid 252 Sp.Att Surf vs 252 HP 252 Sp.Def Calm :
Blissey - 11% - 13%
Wigglytuff - 29% - 34%
Clefable - 25.5% - 30%
That's significant difference.
Blissey can use Counter much more efficiently than the others due to her high HP which means that everything hit by her Counter will die.
Last, Blissey and Clefable have awesome abilities, with Clefable having the best ability of all 3 and Wigglytuff having the worst.
I am pretty sure that Blissey is the best among the three, then comes Clefable and then Wigglytuff.
I mean, Blissey can be used as an awesome Sp.Wall/ Cleric. It can even take physical hits approximately as well as the other two.
Clefable may be used as a Tank or a decent Sp.Wall but she is completely outclassed by Blissey.
Wigglytuff can't be used as a main wall or something. I don't know. She has a huge HP but her defenses don't help much. Also, she doesn't have instant recovery or a good ability. It could probably work as support with Wish.
Does the fact though that Clefable is still able to take a physical hit better than Blissey make her better?
I mean a standard EV spread for Blissey is 148 HP/ 252 Def/ 108 Sp.Def. She can wall special hits all day long and take a physical hit decently.
Clefable's standard defensive set is max HP and Sp.Def . She can take special hits well but Blissey outclasses her seriously. Also with no Def EVs Blissey can take physical hits better than her.
Wigglytuff is not good enough I think and I shouldn't compare it in the first place. Best defensive EV spread is sharing EVs in both defenses. This way though, Blissey can still take hits better in both sides.
Wigglytuff is kinda out due to just passable defenses, bad ability and no instant recovery. It can't be an actual wall.
Actual debate is between Clefable and Blissey.
Blissey takes special hits much better.
Clefable takes physical hits better but not too better(it's a difference of 10% at best). Usually Clefable will only endure one more hit than Blissey. Nothing dramatic though helpful.
Both great abilities.
Both great movepools, great support and instant healing.
I think Blissey is better support though because even though both can be great clerics and supporters, Blissey shares much bigger Wishes.
Who do you think is better?