My 'favourite' drug has changed over time, so I voted my current favourite recreational drug. Which is ketamine, tied with caffeine. I couldn't live without caffeine, whereas in the past 5 years I've only done ketamine twice. I have to be in the right mindset to khole which is the only way I do ketamine, and depression makes having that right mindset impossible.
1.
My first favourite drug was MDMA and if it still worked for me, it would probably still be my fave. I only had a dozen rolls before it turned on me completely.

2.
My second favourite drug was methamphetamine, and if I hadn't decided to stop using it because of how unhealthy it is for my teeth and brain, I'm pretty damn sure it would be my fave now. Meth never turned on me, it was always superpowered, euphoric, and sexy. I still crave it on occasion and I expect I always will.
3.
This leaves ketamine - a drug that I don't use frequently, not that I really do any drugs, been out of that stage of my life for the most part for a decade -
as my default favourite drug that I intend to use for the rest of my life, if only infrequently.
It's worth noting that while I don't find opiates particularly recreational, I expect to be using strong ones daily at high doses for the unforseeable future, possibly forever. It's not worth living in chronic pain unless I can relieve it substantially.
P.S. My list of recreational drugs tried is 40-50 in total...most drugs aren't that fun, interesting, pleasant, or rewarding. A top five for me would include LSD and heroin in addition to the three above. So something like...
1. Ketamine IM
2. Methamphetamine Oral
3. MDMA Oral
4. LSD Oral
5. Heroin IV