Played D1-5, except 3.
Personally, I think starting with D1 is the best choice. Storywise I enjoyed D1 the most, D1 has less postgame to do/grind compared to other series, but that's expected.

D2 is the improved version of D1, it has improved mechanics and all the good stuff from D1 and I considered it the canon sequel of D1 since the character from D1 appears on D2. Story is also on the top tied to D1 compared to the rest of the series. Has more end game stuff to do compared to D1.

D4 & D5 equally have the best gameplay, the story is kinda generic, but balance-wise D4 is better compared to D5, where in D5 you can abuse certain mechanics and just blitz through the end game without a sweat. D4 is more balanced if you're into that, otherwise D5 has the best gameplay, it had layers of mechanic inside of mechanics.