The best thing by far for a harsh anxiety comedown is Xanax, without a shadow of a doubt. Valium and other benzo's are good but none completely eliminate the anxiety like Xanax. I don't use benzo's any more but when I do get very anxious I use Gaba capsules, works on the same principle as benzo's but without the addictive aspects. 500 - 1000mg usually settles the nerves nicely. Better yet you can buy it over the counter. Also for getting to sleep I usually take Olanzapine (Zyprexa), it is especially useful if you have been taking rc's, eliminates any possible paranoia or psychotic episodes. It can bring you down off almost anything if you don't like what you are experiencing. I have used it to come down off lsd a couple times, when I got a bit freaked out, got rid of the nasty visuals I was having and the anxiety that went along with. Have also used it to come down off speed, coke, mephedrone, methylone, mpdv as well as many others. Another magic drug.
Other things I use sometimes to aid with the comedown that can be effective are; Valerian (Liquid Drops), 5-htp, Passiflora, Pregabalin (Lyrica), as well as plenty multivitamins and triple omega fish oils.
Xanax is the king of all comedown cures in my book though. I remember the day after taking a good lot of Bzp, the worst comedown I've ever had in my life, way worse than Mdma as anybody who has taken it will know, we got our hands on some Xanax and each took 1.5 mgs. Within twenty minutes all that horrible, disgusting anxiety and the overwhelming feelings of dread were gone. We couldn't believe it. It helps with any kind of a comedown at all. But try not to use it too much, it is probably the most addictive drug I have ever taken and if you use it regularly the withdrawals from it are disgusting, it borders on stim psychosis, plus one can easily become addicted to it. I don't use it any more thankfully. Gaba now seems to do the trick for relieving that harsh anxiety and olanzapine (15-30mg) gets me off to sleep as well as getting rid of paranoia. Hope this is useful.