That's why I said fans and you've said before you prefer your games to be serious in terms of story as well as gameplay :p
Personally I like P3 more than P4 cause P4 has balancing issues in terms of the high school sim vs. dungeon crawling. P3 was far more flexible and generous with resources so you could play with the fusion system more unlike P4 where you'll be maxing out the part time jobs first and grinding for money cause the game is frugal about giving the player anything.
P1 is old school RPG and has a rather complicated elemental weakness system. When I say old school I need to specify, it may be a PS1 game but its design is more akin to first person dungeon crawlers (with a more PS1 battle RPG battle system) and its plot is downright ridiculous, though the alternate Snow Queen quest is a bit more serious and horror genre like.
P2 has a much better battle system that's pretty similar to Breath of Fire IV's combo attack system. Skills can be combined to make new or bigger attacks and its all based on the order the skills are cast in. The cool part is that you can actually change the parties order to accommodate for slow characters but what this does is make the party unable to move until this slow character gets his natural turn. Meaning you have to be really careful on how you set-up your party and sometimes have to factor in the risk of letting bosses get the first strike in a battle in exchange for pulling off a super powerful Quintet Combo attack over using smaller combos to keep your healer free to heal. it also has P1's complicated elemental weakness system, throw in it has one of the more serious stories in the series once you get to P2: Eternal Punishment.