For me heroin (or other IV opiates) is the most addictive. One dose can hold me much longer than almost any other drug. The euphoria is exactly what I like in my drugs, opiates make me worry-free, confident, witty, energetic (at the right dose), friendly, caring and all around more enjoyable to be around.
It's also addictive IMO because you can keep using it daily with the same effects (as long as you can afford it) unlike MDMA where if you abuse it enough, eventually it'll stop working.
Opiates/heroin put me on top of my game. Invincible. I feel like I can take on the world or whatever the world can throw at me with a optimism, calmness and confidence. It's the feeling of not having a care in the world that is so addictive to me. No other drug I've tried can match the consistency of euphoria of opiates.
IV coke is nice, but the high is to short and the crash is too hard. Stimulants aren't for me...I just don't enjoy them, except MDxx's. Nicotine is quite addicting but it's not even in the same league as opiates. Booze is nice but it makes me too depressed. Cannabis used to be very addicting to me until the paranoia outweighed the euphoria. Opiates FTW!