My start with the franchise as a whole was SMT1, and personally, I never regretted that. It was a lot of trial and error, it could feel clumsy at times, but if viewed reasonably as a product of its time, it can be a lot of fascinating fun on its own. It's given me an appreciation (and perhaps, bias) towards 90s Atlus games a whole, admittedly.
SMT3, and 4 make for somewhat refined experiences in terms of gameplay, though they can still unprepared players' butts, one should note.
(in further terms of 90s Atlus gameplay, I think Soul Hackers refined it to a, uh, well, fine point.)