As far as stims/entactogenics/empathogenics go; Mephedrone takes the cake, followed closely by MDPV (flushed that shite!). The mephedrone isn't so moreish when bombed, but snorting is a completely different story, haha. And MDPV gave my psychosis once (like schitzo to the max, thought the feds were following me psychosis!), then it just turned into a boring ass stimulant that made me sweat a lot more than, say methylone would, so I promptly got rid of that. Didn't help my studies a damn bit.
As far as downers go: I had a long run with bali kratom addiction (yes, there is such a thing when you don't bang H or take hydrocodone, etc., which I don't) but I've learned my lesson after some nasty GI issues

. Now I limit that to once a week or every three days and it's much better that way. As far as benzos go, I'm scripted clonazepam and temazepam. Temaz is MUCH more moreish than the k-pin so I alternate. Fuck benzo habituation :[ (Semi-Trying to ween off with kava and phenibut)
Psychedelics tend not be moreish, but 5-meo-dalt with mephedrone makes for quite a long, crazy ride and I tend to dose more meph than I should when doing that. Never ordering mephedrone again, too unpredictable.
That's my two pennies.