Agreed that Persona and SMTIV are the best gateway drugs. Go with IV if yoi want to jump right into SMT, or try Persona 3. If you like P3, try 4 and then work your way up from 1 to the two 2s. Cant comment on 5 yet.

Soul Hackers is also a pretty goo starting point as something kinda between Persona and mainline SMT. It has a lighter tone and is part of he Persona timeline, but has all the mechanics of mainline. Its super fun! Or, if you want somethng closer to a more conventional JRPG, try he Digital Devl Saga dulogy