No, what I mean really, is that weed by itself (or god forbid, mixed with stims), induces a lot of anxiety and paranoia for me. When combined with heroin, then yeah, it's great, as it seems to just make the heroin high stronger.
Ah ok. Yeah, I know a handful of people who don't toke often or don't enjoy it because of it's ability to induce anxiety. Really not a uncommon thing, but is very tragic. It has gotten me before, but not quite like what I've seen it do to others. The more frequent I smoke the less likely it will happen, and the less sativa dominate the less anxiety will be produce as well. Set and setting is also a very important key. If I smoke a sativa dominate it's best for me to be around a single good friend (or two) in a similar environment, and a simple spoon as the smoking device (slower build up of blood plasma concentrations/better control of amount). Some times just taking half a hit every 15-45 minutes till your body adjusts to the high is the best way to avoid anxiety if one chooses to smoke (I use a similar technique for smoking DMT to prevent large amounts of anxiety*). Avoiding bongs, other water pipes, and rolled smoking devices are best as well.
I apply these rules to of smoking while on psychedelics especially. On higher doses of psychedelics mixing even a single solid hit of weed can create a chaotic experience even to the best of us.
*for DMT I'll start off with a 15mg dose, wait 5-10mins, dose another 10-20mg and wait a similar amount of time, and when I decide I am in the right head space/adjusted to the mental physical feeling, I'll blast off with full dose.
/rant (sorry, a little amphetamine induced post)
To the question at hand.
Honestly, I'm having a hard time finding a drug I hate. I would say coke, but there have been times I've had a great time, as well as wouldn't turn down.
At this point I avoid taking diphenhydramine even with opiates due to it's nasty ability to make my RLS hell, especially in my upper extremities. Once the opiates effects run down I can notice a little bit of RLS that the opiates should eliminate. I much rather take doxylamine anyway. Less psychological effect that can be a bit dysphoric, and no RLS intensification.
I'm not a big fan of amphetamines, but I don't have access to meth/don't want to deal with shitty meth/meth heads. I don't mind a good stimulant high, but amphetamine (even dextroamp), cocaine, and methylphenidate, aren't that amazing. Good meth on the other hand is awesome, but I really don't have too much experience to make the most accurate judgement. I will say, oxymorphone + desoxyn was one of the best drug experiences of my life. Ketamine can also make amphetamine one hell of a drug (k + amp, better than MDMA). So I guess really, I can't even say amphetamines I hate.
2c-i has to much of a body load for me to ever want to really try again, and even at high doses, the psychedelic experience isn't worth it. Not a terrible drug though.
Alcohol sucks a big dick, but is convenient and I have had some amazing times. If I have other drugs I will avoid it at all costs though, so might pick alcohol as my least favorite.
oh and tobacco sucks but if it werent for it then we may have very few legal ways to initiate conversations 
