While actively using, stimulants with a very strong compulsion to redose, like cocaine or MDPV, are the hardest drugs for me to stop abusing but only in that moment. Once the binge is over, it isn't all that difficult for me to not start using again. It's just really hard (nearly impossible) for me to stop during a binge while I still have more in my possession.
Opioids are easy for me to stop once I've made up my mind that it's time to stop. Getting to that point, however, can take forever. I don't feel compelled to take them, the way I feel about stimulants during a stimulant binge. I only keep taking opioids to fend off withdrawals. Once I decide enough is enough, it's over.
With stimulants, I never decide enough is enough if I still have access to more... I either run out, flush my stash in a paranoid frenzy, or I wind up getting myself locked up in a mental hospital for a few days...