Ok of what I know for the mighty majin ultimate is better because it has better buffs so you'll be in great demand for dungeons and such. I know chefs do have a buff that gives 20% more con which is great and they also have stronger skills and AoEs and something similar to a spin attack like ultimates do.
For the wonder majin the karma is one of the best PvP classes because it has many Knock downs and stuns and debuffs and also good attacking speed buffs which is also really wanted in dungeons for speed parties and such.
The plasma has some really good AoEs and some cool debuffs but I don't think you'll be really good in PvP for 1vs1 but in 5vs5 maybe. And for dungeons I don't know if many do like to take plasma along. So maybe karma is better.
This is pretty much what I know hope this helped. I would also recommend looking at the skill calculator and see what class you might like better based on how soils look. Since you are gona be playing with it for a while. good luck.
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