Shrooms would be my favorite, speed my second favorite, but I've yet to do quite a few drugs. I've done speed a lot more than shrooms so I'm gonna go with that.
Oh and its speculated that small quantities of DMT are released during REM sleep (dream-state) but I don't think research points in the direction that DMT causes your dreams. Isn't the molecule real similar to adreneline or serotonin?