Honestly, I'd have to say cocaine.
Reason being, its the only drug that I lose my rationale on when it comes down to redosing.
For example, if I'm using meth (i love meth), I always have a time where I don't redose anymore. Almost like a boundary line. If it passes 2am, I won't redose, because I know I won't be able to sleep.
However, if I'm using coke (i love coke), I set a "boundary time" for myself, but when that time comes, for some dam reason I think "meh, another line wont do any harm, it doesn't last as long as meth does"
But you see, that just keeps going and going and going until I run outta coke - then I won't use it anymore.
But god dammit, when I comedown on coke I fiend H A R D.
SO HARD.
I get angry, i get down. Like, I've never felt that way before.
So, coke is the most addictive for me (even though I like meth way more).