SMT DS Overclocked is still, to this day, my favorite SMT ever. I think this game is what got me into visual novels because of all the choices and consequences you make along the game, the secrets you can uncover and the endings that felt very satisfing to each route.
It's probably also the game that also got me into SMT because of the huge hoster of demons, simplified extra turn mechanics and fast gameplay loop. I still remember the secret boss in the final day that made the rest of the game look like a cake walk, that was probably the first boss that made me sweat while playing a video game.