I would say that even though MDMA has gained an awful rep as a neurotoxic brainholerater (yeah, I made that word up all by myself!), it can still be a wonderful introduction to the wide world of psychedelics AND a lot of research shows it is not neurotoxic in any way at normal recreational doses, kept to a maximum usage of once per 3 weeks / month. The case for MDMA's non-neurotoxicity is made here by the well-informed Nathan Luno:
http://thedea.org/neurotoxicity.html. New research, he adds, (the so-called massive NeXT Study being done by the government of the Netherlands) confirms this: " Every test to look for the sort of brain injury that has been seen in lab animals failed to find any sign of damage in the people who had used 'ecstasy.' "
But why is Ecstasy such a good introduction? First of all, a bad trip is almost impossible -- I think most people would consider a psychotic experience quite "unsafe." Second of all, it isn't called Ecstasy for nothing -- it's fun if nothing more. Third, most people dabbling in psychedelics are looking for that "something more," and users habitually experience a deep, profound connection with the world and its inhabitants without the possibly alienating effects of a stronger psychedelic like LSD.
I concede, however, for a true full-on introduction you should probably try shrooms, because they're very safe, have few side effects (except nausea sometimes), and don't last for as long as LSD. With shrooms, you'll get the nice visuals and the pretty patterns that you might be looking for. The "big 3" to try, IMO, would be shrooms, LSD, and MDMA. They're all fairly safe and honestly, if you're going to enjoy psychedelics, you should probably be ready to take some risks.... your mind is a big place to explore, and there might be some pitfalls.
PS-As a dissociative, couldn't ketamine be rather scary for a first-time user? I've seen people in k-holes and they're not always loving it. But if you have experience with Salvia, another dissociative, then maybe you'd be good.