Ketamine is pretty addictive, but not particularly functional.
To be honest I don't really see any correlation here. Psychedelics are some of the least addictive drugs around, true, but provided you aren't taking heroic doses plenty of them are quite "functional". On the other hand, there are plenty of functional opiate addicts, alcoholics and even daily ketamine users who manage to get by just because they aren't taking huge doses while they have to do stuff they have to do. So dose is a big big factor here and really muddies the waters of any correlation you try to draw.
Stimulants obviously would be the most "functional" class of substances by default, in that in measured doses and non-excessive use they are generally performance enhancers, so by this measure stimulants should be some of the most addictive drugs around, and maybe there is an argument for this. But dose too high or too often and any performance enhancement along with a big chunk of your ability to function in day to day life goes out the window... on the other hand I would argue that opiates, benzos and sedatives in general really are not very functional at all - again, people are just able to get away with using them by controlling their dose.
One final point - I think the examples you've given are pretty arbitrary, as far as weed and LSD being the most disorientating and least addictive. Also, what are you saying is worse? Operating heavy machinery on a few benzos, or after smoking a few joints? To me the answer is not at all obvious, even from the context of the rest of your post, but again, it's going to be almost entirely dosage dependent.
