Aye, I would really rec Megaten too. But like there are so many games under it that it is kinda wild. The mainline and Persona is good and all but my personal favourite is Devil Survivor 1. That game is a work of passion. Other side series that are often praised are the Digital Devil Saga and Raidou games but I havent plaed them so dunno.
I was obsessed with unlocking all the endings of persona 1-4, and devil survivor 1-2. It's REALLY fun, with multiple endings and choices that you'll make will truly impacts the ending! I think desu 1 is better than the second one though, since it offer more choices than its sequel, although i still got the new version of desu 2 (the limit breaker one). Another personal favorite are tactics ogre : until death do, us part with its loads of replay value, suikoden 2, and yes, trails in the sky.