If it's a party where I can feel open and safe and there are several people I know there, then at least LSD. MDMA and mushrooms are a nice plus, but just LSD is fine, as long as there's enough. If it's at a very public place, I may dose a bit lower, but in psytrance parties where I know lots of the people and feel comfortable being a little spaced out at times, milligrams may be appropriate.
LSD was never a party drug for me until I felt that I'd exhausted most things I could learn from it through introspective tripping. I'd experienced the dissolution of my ego and vividly agreed that my life had ended - and resigned to my fate. At some point I realized my trips, regardless of set and setting, were becoming repetitive in their content. So now I choose to trip with friends, and learn new things about them, or appreciate the beauty of their thoughts, their words, or new surroundings and landscapes; or then at parties, hopefully with a few friends looking out for me so I can just gobble down a handful of hits, lose my shirt in a corner and let loose.
I've done my fair share of lying in the darkness, philosophizing and imagining complex grammatical models of concepts. If I want introspection, deepened knowledge on metaphysics, or simply information or influence from ancient, even alien beings I can hardly begin to understand, I'll take psilocybin or (preferably) DMT.
So strangely enough, LSD is now mostly a party drug for me. 12 hours of energy, music not only sounds awesome but... tastes, feels, looks, and smells awesome, and when you're tripping hard enough you'll think of at least 6 other ways to perceive it. Dancing is not voluntary, nor do I consider that I dance; I am a vessel, a cog in a machine, compelled to move by the musical circuit of raw psychedelic power. Fuck yeah. %)
If LSD is not available I will almost demand amphetamines, and though they are tolerable on their own, they leave me very sober (and unable to get drunk) so I don't really feel any better - just sweatier and with about 6 hours more of dancing in those legs. Add in MDMA and that's a good party state, but it completely pales in comparison to candyflipping.