Overall - given the shittiness of the high and length of the after-effects, I'd say the worst comedown, in general, is from piperazines...but while the high is infinitely better, speed of any kind is just as bad...especially if mixed with acid. Sorry for the length of the anecdote to follow, but it was one of the most intense drug experiences of my life...
My own worst-ever comedown began in London - 2 hits of good X and a little amphet sulphate at a club night, followed by rapidly smoking 1/16th of hash - before flying to San Francisco, via Newark.
Took an Ativan, drank on the plane - woke up in a room at Newark airport, being interrogated by two cops who'd found me passed out on the floor. They confiscated my bourbon and let me go, so I fly to SF....
And immediately buy a half g of meth, not knowing how much stronger it was than the sulphate I was used to - and did the whole lot, other than a few lines I gave away, in OJ. For some reason, I bought some acid - had two hits secreted in a bag in my mouth, walking back to the hostel I was staying in in Soma - and a cop-car buzzes me, slowing down to get a closer look (I was a longhair back then, and a deadhead, so cops tended to eye me closely). I panicked, and swallowed the bag of acid (pointlessly: the cops drove on, didn't stop me). My stomach was so acidic from days without food that it burned through the plastic bag the blotter was in. 24 hours later - after a night of amphetamine psychosis from the meth (black helicopters following me, imaginary cops everywhere, the whole bit) - two hits of Haight-Ashbury blotter start to hit me. 48 hours sleepless, full of meth, sulphate resdue and serotonin burn-out from the X...suddenly tripping without booze (I was only 19 and new in town), benzos, or much weed - and it wasn't really helping at that point. A bowl just span me out even further.
Needless to say, the trip was the most hellish LSD experience of my life: with my dopamine levels jacked up from two days of speed and X, maybe something like a schizophrenic might go through if they dropped acid. I was convinced I was going to die, was terrified my room-mates would rape me, and soul-destroying visions of cosmic emptiness had me weeping for hours.
The comedown was pure hell - 12 hours of panic attacks, persecutory delusions - at last, a dorm-mate bought me a bottle of Southern Comfort, and I managed to choke half of it down from sheer desperation (hate that fucking stuff, haven't touched it since). That, along with 3 sominex and sheer exhaustion finally knocked me out for a few hours. Physically, I shook like an alkie with Dts.
Luckily, the hostel was pot-friendly and I got an intro to the house dealer - I spent the next three days in bed, smoking pipe hits every time I woke up till I could sleep again, occasionally stumbling out to buy some junk food, and once luckd into some black tar, which brought much-needed relief. Remained anxious and paranoid for weeks afterwards - and never touched meth again, which was definitely for the best, as I went on to develop a problem with dex and bootleg amph sulphate (which is mostly what 'speed' means in England).
Next worse was from a single pill - bzp/tfmpp mix - that didn't bother getting me high, just gave me a 24-hour speed comedown.
Shit like the above may be why I've given up stims completely, and now use only weed, downs, painkillers and the odd dissociative. I'm waaaay too old to go through those endless, agitated, sleepless nights again.