The main series, largely because Nocturne/Lucifer's Call is my favourite ever game.
The Persona series is actually my least favourite SMT sub-series, though I still love them (well, except the first one...). I find the calendar system in P3 and 4 inflates the lengths of the games beyond reason; I was clearing eachg dungeon/block within 3-4 in game days, yet I had to wait a month for the story to advance. In the end, both games took me 100 hours to beat, but it didn't feel like they had 100 hours worth of content. And the social links seem cool at first, but the fact that there's correct choices for advancement and only one path to go down is a little annoying. I'd have preferred fewer links with branching paths. And maybe not have every single one of them being about the MC solving their crippling life problems then having them fall in love with him (if female).
Also, with the a few exceptions, I'm not fond of P3 and 4's soundtracks (or the PSP soundtrack to Persona 1, though the PS1 OST kicks ass). P3's January town theme and the final boss theme are awesome, but the Burn my Dread, and the P4 battle theme are almost unlistenable to me. I just don't like that kind of music at all.
That makes it sound like I hate them, but I don't. They're fantastic games, but the main games combine far superior gameplay with storytelling I find more interesting, better atmospheres and far, far, far better music.
Persona 2 is great and has my favourite story and characters out of all of the Personas and MAYBE the franchise (certainly characters, unsure about story). The gameplay, however, is very antiquated and wasn't that fantastic to begin with. It gets better as you progress and unlock more Personas, especially since every character can switch as opposed to just the MC, but some may feel it's too little too late by then. Persona 2 also has a fantastic OST. https://www.youtube.com/watch?v=l3pDwxdAWHY
Digital Devil Saga is great, too, but the story doesn't really get going until the second game, imo, and though it retains the awesome battle system from Nocturne (and even refines it by not causing a game over upon the MC's death) the lack of demon recruitment and less interesting dungeons hurts it in comparison.
