Kirai- Go through walls, Summon lots of stuff, Easy to die if caught off guard, Absolutely impossible to die if you anticipate the attack.
Dreamer- Switch back and forth, Crazy combos, Not enough ranged attack, Doesn't drop any corpse counters, Surprise Melee Monster
Ramos- Builder, Summon little spiders/electrical creations, Army is mostly melee based/Ramos can add some ranged support, Suicide bombers
All are about equal in effectiveness. Ramos and Kirai would be easier to expand on later, but the Dreamer is still a good choice.
Personally, the master I would choose depends on which masters my opponents at my local game store have :1244: