Natural: Mushrooms, because I have the longest, closest relationship with them, and they gave me my first three breakthrough experiences, which were honestly more sublime and meaningful to me personally than anything I've experienced on DMT thus far. But I must say, I feel one day the answer to this might be mescaline... as it seems to be for so many others.
Synthetic: LSD. Actually the only synthetic I've ever tried, but I really love it on multiple levels and have had quite a few incredible experiences on it that I can't imagine being topped by another.
Vi veri universum vivus vici