Hard to say. Goa / Psy Trance is a really broad genre and it's been constantly changing over the years. Generally we could say there is stuff that sounds more "old school goa" or "full on" and newer stuff, like "minimal" or socalled "techtrance". Now we could go into this forever and still a lot of people would disagree as discussing goa/psy subgenres is always highly controversial.
I've been into goa for over 5 years now and still know far from everything, but at least I can give you some hints from my personal listening experience. Further than that I can really recommend
www.psynews.org, an extremely informative site with a great forum.
There are some artists you really can't go wrong with, as
Cosmosis: Albums: Cosmology, Synergy: Imho pure psychedelic trance at it's best, kind of genre-defining for me.
Hallucinogen: Albums: Twisted, The Lone Deranger: Maybe THE most highly regarded artist in the scene, really unique sound and composition.
Infected Mushroom: Their first two Albums "Classical Mushroom" and "The Gathering" are considered real old school classics, personally I really like their 3rd Album, "B.P.Empire" which I find extremely appealing in every aspect.
Pleiadians (aka Etnica aka Cropcircles): Albums: Identified Flying Object, Family of Light: Two really stunning albums with a distinctive sound, though different in execution. While IFO is really straight full on until the last track which is down tempo FOL presents more diversity between songs.
Other Names worth mentioning:
Astral Projection: kind of defining the "old school" sound
MFG: old school goa, but different in sound
Transwave: again nicey old school goa
Atmos: very atmospheric sounding goa trance (hence the name)
Toi Doi: fast, hard, psychedelic trance, maybe "a bit too" full on (depending on mood)
Midi Miliz: dark stomping tech trance
The Delta: see above, someone once called it "gothic" tech trance
Logic Bomb (aka Hux Flux): very unique sounding psy trance, especially Hux Flux having a high "weirdness-factor"
Juno Reactor: considered goa, but not really trance; Personally I consider their Album "Shango" their masterpiece (which is a kind of electro/world music mixture)
I guess that's it for now.
Have fun exploring!