I've been friendly with the Shin Megami Tensei series for over twenty years now, since the release of the incredibly crappy localization of the original Persona on the ps1 (believe me, it is one of the worst localizations of all time). That said, I saw the series as just a darker than normal jrpg series... until I played SMT: Nocturne for the PS2.
Nocturne is frequently referred to, both seriously and derisively, as 'Pokemon with demons and a cohesive story'. Seriously. While the Persona series has some of the same atmosphere (collect all the Personas! lol), it is Nocturne that introduced me to the extreme difference between the series and the common ruck of jrpgs out there (which were almost universally swords and sorcery at the time). I was blown away when, in the first half hour of the game, the world is destroyed, turned inside-out, and the protagonist gets a centipede-like bug planted in his eye, giving him demonic powers. To say the least, this was an... unusual turn of events in my experience. The game wasn't about saving the world... it was about determining what came afterward... and everyone in that world wanted you to jump on their bandwagon. The game also introduced me to the staple scenario of the series...
****WARNING, the following is offensive to some of the more sensitive religious types out there****
God was the enemy, Satan is your would-be mentor, and saving the newborn world to come involved starting a rebellion against the established order.
To say the least, I was shocked. I mean, as I dug deeper into the optional dungeon (which is how you access the true ending), I was forced to a realization of just where things were going... and it was more than a bit of a shock to the system. The game itself was enjoyable, and it was the very first game I literally leveled up to the max... and still had trouble with the final boss (lol). It also introduced me to the harsher battle mechanics of the main series, which was the main reason why the 'in-crowd' tended to refer to the Persona series as 'Kiddy-Tensei', both for the less mature themes and the more brutal difficulty levels.
The next two shocks to my system were Persona 3 and Digital Devil Saga... Persona 3 hit me just as I began to take an interest in VNs on the periphery of my vision, so it is no surprise, in retrospect, that I enjoyed it so much. However, it is Digital Devil Saga which, in my eyes, still represents the best qualities of both sides of the SMT series. It had the high difficulty levels of the main-series games, along with a story that still, even after I just finished P5, leaves every other game in the series in the dust. It was dark, interesting, and brutal in the extreme.
Persona 4 was kind of a letdown after that high... though it was still good. To be blunt, when the original version of P4 came out, my basic standard for SMT was DDS, period. P3 was, to my mind, an interesting game in its own right but inferior in comparison, despite its social links. The somewhat goofy nature of some of the characters (the party members) only emphasized that attitude on my part. Persona is the only SMT sub-series that tends to make me feel like I'm playing a 'normal' jrpg, albeit one on a tight schedule. Understand, it is all relative, in the end.
Last but not least, we come to my most recent experience (SMT IV was something of a dud in comparison to Nocturne, so I'm ignoring it), Persona 5. Persona 5 embodies both the best and the worst of the last three Persona games. It allows you to form deep personal bonds with various interesting characters (ironically, the non-party ones are much more interesting than the ones you fight with, for the most part), and it also manages to combine imagery of rebellion and imprisonment with deep themes of self-determination and personal justice. In addition... it shows rather blatantly the worst aspects of the Japanese legal system (for those who played the game... yes, Japan's police and judicial system really can be that messed up, if you get on its bad side). Japan is a country where almost all convictions come from confessions and plea deals... that should tell you a lot about what it is like behind the scenes. Not to mention that it is a country where it is extremely hard to argue self-defense (if you give someone a defensive wound, you can be sued), insanity defenses earn you permanent social stigmatization, and even a single smear on one's record can lead to a permanent inability to get any job that pays above minimum wage outside of day labor.
As a game, it makes some improvements on the Persona formula as defined by Persona 3... in particular, the benefits of a social link are more clearly defined and useful in the game. The Tower Confidant, especially, has an ability you'll be extremely happy to have when hunting rare personas.
Story-wise, it is at its strongest near the end. It is sadly predictable throughout much of its length, in comparison to 3 and 4, and the last boss was not that hard to predict given the tendencies of the series. That said, I honestly felt a much larger emotional connection to even the most annoying characters (all of whom were party members, incidentally... which is probably the worst aspect of the Persona series) than I did in 3 and 4. I played the game without using walkthroughs, and as a result it consumed ninety hours of my time to finish and I missed finishing three of the Confidants. However, I find myself feeling rather satisfied, over all. The way the last part is done, however, gives me definite feelings they'll probably do an FES-style sequel. They simply left too many openings for it, and, while the big bad dude is no longer around, it isn't like the SMT universe is the kind of place for pure happily-ever-afters...lol