GOOD MDMA. Not cut, stepped on, speed laced "molly" or E bombs. I mean some good, clean, pure MDMA. I never had heroin or methamphetamine but can't quite possibly imagine something more euphoric than this. Granted, the serotonin storm euphoria is much different than a more dopamine based one, but personally that feeling just cannot be touched. I'm sure others may prefer the dopamine hit.
Mephedrone comes in a close second. Last time I took ~200mg of it, it hit me while I was on the dance floor with my girlfriend and felt so good that I was actually worried I might come in my pants.
However, neither of these is practical for a 'mood lift' where one plans to go about their normal day to day routine. They're great for partying, but I don't think busting a nut in a meeting at work because my drugs hit is a good thing. For this situation, I say nothing is better than a strong cup of poppy pod tea. Strong, clean euphoria that lasts easily for ~24 hours, maybe longer, without too much 'intoxication'.