>>725521273
For sheer entry level?
SMT1 & 2 SNES or PS or GBA is essential.
SMT3 emulated or remaster, whatever you prefer.
SMT4, some may argue try Apocalypse, I'd argue they try lobotomy.
You may as well Strange Journey while you're at it, there are strong opinions about Redux as well but that one I'll leave to you.
The obscure stuff mentioned here about events in NINE requires emulation or patience with a video, because I doubt browsing the wikis will make a lick of sense.
While you're watching vids you may as well watch the DDS Megami Tensei and Shin Megami Tensei: Tokyo Revelations anime movies. They don't provide a lot of context for SMT though, the first is the premise for the Megami Tensei Games you can play as the Kyuyaku Megami Tensei compilation which has elements that SMT references or uses but isn't vital to know, the latter movie is actually closer to Giten Megami Tensei but you probably can't play that, even so it's still interesting to watch. Very abrupt ending though, I'll warn you.
For a similar vibe see if you can find the Devilman OVAs while you're at it.