If you really want to do this, then you should also weigh in the times when you don't have access to the drug you want. The withdrawal symptoms should be your main concern in the long run, because believe me, after some years you will completely forget how good they used to feel and you'll start using them just to escape the negative effects and issues that they cause.
In my opinion, the withdrawals in a descending order will go like:
- alcohol/benzos
- Heroin and heavy opioid use
- cocaine
- meth and similar stimulants
Whichever you choose, just avoid alcohol and benzos like the plague, this is now being accepted as the worst drug related withdrawal because it can literally kill you and some people argue that the GABA receptors can only partially recover, if they can recover at all, so you're looking at years of detox and life-long side effects depending on your level of abuse.
They will also turn you into an anxious mess really quick, more-so than any other drug.
Btw, don't take this as me trying to stop you, it's important to know this stuff before making a choice.