Definitely kratom. I've never tried tramadol, but the serotonin/NE/NMDA action seems a bit shady. Also, opinions on tramadol are rather mixed. I've done kratom and I am a huge fan of it in fact, it has been able to provide me with very euphoric moments - shame it's been made illegal here. It (or maybe it just accompanied the situation) changed my life as I made quite a few very radical decisions in my life under the influence of kratom that I have stuck by until this day, and they were for the best as far as I'm concerned. I'd take kratom over many drugs, even over many opioids. It's one of my favorites. It actually brought a tear to my eyes thinking about the times I was using kratom.
Unlimited access to a long-acting and long-to-take-action opioid like methadone or a fast one like heroin? You have to consider the withdrawals too.