Opiods such as opium, codeine, heroine, fentanyl are very addictive, you would understand if you tried on a recreational basis. It gives you a feeling that everything is good and happy in mild form and a euphoric rush if a bit stronger. Takes the pain away too, hence it being used in the hospital.
MDMA may be similar chemically to yaaba but from personal experience it is not super addictive. A lot of care needs to be had taking it though.
Ketamine is not an opioid, it's a dissassociative, and its effect is very different; at low dosages it helps you enjoy the party and puts the beats in sync, at higher dosages it is somewhat psychedelic and you can experience yourself leaving your body, it's a bit hard to describe and different variants give different effect. At high dosages its numbing and you don't feel pain, hence it being used during surgery.