AMT by far, I was addicted to that one at point, thought it was the best drug ever (and still probably do). Noticed in another thread on another forum that some other people were also addicted to this one. Just a beautiful chem, shame that you have to pay with it with your body.
Edit: Ketamine, methoxetamine were never that addictive for me (in the sense that it would be hard not to redose like with AMT) but sure MXE and K are "addictive" so that you're not sober, and it feels good. Though AMT had a strong push because it was just damn awesome, and mind-warping enough to make me take more. The time-dilation helps, but after a long binge on AMT I had "problems with reality"; made me stop.
DXM is another addictive one, absolutely love it; went on a mad spree to get more, but again it's self-limiting. With AMT I had my first taste and thought this is it, have to get more; got 1,3g freebase and tripped for maybe a week. Destroyed my stomach though. Must say though that DOI and 5-MeO-AMT give a feeling of a "big ball of serotonin" on your head, very pleased with those also.
Edit 2: Sorry for any misjudgments, I am drunk.
Edit 3: MDMA is shite compared to AMT; mixed AMT with MDMA crystals (80ish percent) couldn't even feel the MDMA (though I had only maybe 100mg MDMA). Meph, meph, meph!