Decent doses (4-10 hits, at least 300-700µg) of LSD. And I mean real LSD, not DOB, DOM, 2C-T-7, or some other bullshit.

I find it especially enjoyable when combined with a hot bath, sex, or wandering about the city at night chatting up random bums and such (disclaimer: don't try going outside on LSD unless you've got someone sober with you, you've tripped a lot, or you enjoy freaking out really bad).

IMO, once you're used to the effects and can manage the trip LSD intoxication can be extraordinarily pleasurable -- much more so than, say, MDMA or Psilocybin. YMMV.