I'm not gonna talk about withdrawals or anything 'cause that's a slightly different issue.
The worst comedowns I have experienced are probably from crappy ecstasy pills or Bromo-Dragonfly. After a 2-day mephedrone/methylone binge, the comedown was fucking awful was well, and re-dosing made it way, way worse. I had waves of dysphoria. Mephedrone dosed once never had a "crash" for me - it just suddenly dropped from massive euphoria to mellow, reminiscent feelings. You wished you felt like you did 5 minutes ago, but you still feel alright.
I'm not sure if I'd consider an alcohol hangover a comedown or a very acute withdrawal symptom, but if that's a comedown, it's up there as THE absolute worst. Nothing feels as bad as a fucking slamming hangover. Immobilized for the day and still awful and depressed the following day.
As for good comedowns? Well, apart from drugs that slowly wean off and can't be considered not to have a comedown (like cannabis, benzos and the like), acid is the smoothest - it just mellowly plateaus, although there is a very swift and dramatic change in how intense and psychedelic the trip is. You just suddenly realize you're not quite there anymore. You can then smoke cannabis to relax and slightly "rewind" the trip, or eat benzos to relax more and possibly gain sleep earlier if you don't wanna wait as long. You'll probably feel really good if you get a good night's sleep and wake up in the morning - it is the very opposite of a hangover. Many, myself included, also report that smoking weed the day after an acid trip strongly brings back feelings and sometimes visuals from the day before.
Ecstasy comedowns depend on pill content. They can be awful or tolerable but bad, if they're shite pills or pure pills - either way, I prefer to withhold my pills (unless a party is coming they're intended for) until I know I have at least cannabis and preferably benzos for the comedown. I don't like the comedown from even good E, although it is sometimes rather smooth.
On acid comedowns I will take benzos very late, when the psychedelic effects are almost gone, but I want to sleep, usually around 9-10 hours from when they started to work. Weed I'll smoke a shit ton of, as much as is available. With pretty much any other comedowns or hangovers, I will take benzos and cannabis as much as possible and they relieve almost any awful feelings.