Benzos are probably the worst in their acute state, but at least you can get around most of the suffering by a very, very gradual taper. I've been weening off for almost a year, down from 3-4 mgs of Xanax daily, to a switch to Librium, which I switched to after getting down to 2mgs xanax daily and hitting a wall (my idiot psychiatrist tries to ween xanax addicts with xanax, and tries to do rapid detox, making him totally fucking ignorant and incompetent). I started at 100 mgs daily, which is roughly equivalent to 2mgs of xanax, and the switch has been the hardest point. I am now down to 10 mgs daily, with a 5 mg capsule in the morning and a 5 at night. I detoxed in increments as high as 12.5 mgs a month (by taking two pills during my night dose one day, and one pill the next, then two, then one, etc etc..), and now I go by 5.
It really isn't that horrific if you take it as slow as possible. The worst part of my detox is seeing my psychiatrist every month and have him berate me for "taking it too easy on myself" (literally his words) and his attempts to have me try and cut down on my Suboxone at the same damn time. This guy is out of touch, he is an addiction specialist who hasn't read the Ashton Manual, what the fuck. He has no concept of pain and suffering through withdrawal, and he should be okay with whatever the fuck rate I want to go, provided we have consistent progress, which we do.
Anyways, that makes opiates the worst for me. I'm thinking I would like to get off Suboxone, but withdrawal at home seems impossible. I can minimize pain with benzos, but with opiates you just have to bite the bullet if you can't get into a detox, which I am not doing for Suboxone (couldn't afford oxy/heroin detox, much less a 21 day suboxone detox that usually leaves people in withdrawal when they get out). Post-Acutes are a terrible thing as well. I know that the shock exhibited by benzo addicts who are rapidly-but-safely detoxed by ignorant clinics (these places want to treat every depressant like opiates sometimes) is worse, but at least you can avoid them. There is no true way around Post-Acute Opiate Withdrawal, and the subtlety of the symptoms is such that it becomes hard to determine whether the depression is simply how you feel without opiates (the lie) or a direct result neurotransmitter imbalance due to a chemical substitution (the truth). You can only remain depressed like that for so long before you start to believe the bullshit.
So, while all out benzo withdrawal is worse, opiates are much more difficult to withdrawal from, as nothing you do can make it anywhere near painless (even detox is rough I understand from lots of second-hand experience, although I've never been). That leaves opiates with the edge as far as I am concerned. Post-Acutes no matter how well you withdrawal are the absolute deciding factor.