Id have to say MDMA for sure, as long as its kept to a minimum, ive had periods of my life where id be doing MDMA between once a week and once a month and for awhile it was worth it then I started losing the magic. Now I only use pills maybe once every 3 months, at present I havent done it in about 6 months and that first time after not doing it for so long is absolute bliss. Thats feeling where your lips are quivering, eyes almost rolling into the back of your head and getting shivers of pure euphoria running through your body is amazing. Especially with friends and loud dance music, raving is a perfect example.
Thats only if im around people though, if im alone the best feeling is after a good shot of meth, not much beats that. If I get a good trip acid can be awesomely euphoric too but its not always garaunteed like it is with meth or MDMA. Opiates used to be really euphoric, especially a shot of morphine but its not worth it anymore, it doesnt feel as good as it used to and no matter how long I abstain from using my tolerance never goes down, I could even say opiates suck nowadays.
GHB is nice too, lol I cant just choose one, with drugs its usually a case of I like it or I dont, and most of the drugs I like are on par with each other whereas the ones I hate usually have levels of how much I hate them. Phentermine probably being my most hated substance out there, disgusting stuff that is.