This is a tough one. Some of my favorite experiences have been on LSD. In fact, even given horrible setting/situational conditions, ive still always maintained an awesomely positive time overall on LSD.
But im going to have to choc it up to Ketamine. It is the most phantasmal for me. It gives me the greatist feeling of removal from reality (at higher doses) than any other drug I have tried. I enjoy encountering a state of mind so alien that it can not be linguistically described. For me, ketamine provides the most alien of states. Also for versatility. Ketamine can be used just to get straight fucked up and chill with your friends. You can just keep bumping it all night and just have some recreational fun - you dont always have to shoot for a full blown K-Hole. And thirdly, for safety. I understand many psychedellics can not possibly harm the body without one attempting to do just that. The property of ketamine's recreational dosage range being far, far below the medicinal range makes it unique from all other drugs. This uniqueness provides great personal safety (provided the environment allows safety for incapacitated persons).
Ketamine does not provide me with as great as a learning experience as most tryptamines, or lsd, etc. Mushrooms and LSD have certainly made me grow much greater and deeper as a person. So in many respects I would have to say LSD because I certainly would not be close to the same person without it (imo in a bad way). Ketamine however has always been the one consciousness altering catalyst I continuously fiend for, and ive done just about every drug there is.
Its a tough call, it really is. Can i vote 2x please? :-D