Though in Japan there's only like 5 games with the actual title Shin Megami Tensei attached, it's an entire universe/franchise of awesome role-playing video games. In North America they all have SMT and the Pentacle attached, but America is dumb...
So far I've played:
Shin Megami Tensei (NES)
Shin Megami Tensei - Strange Journey (NDS)
DemiKids - Dark Version (GBA)
Devil Survivor Overclocked (3DS)
Persona (PSP)
Persona 2 - Innocent Sin & Eternal Punishment (PSP & PS1)
Persona 3 FES (PS2)
Persona 4 (PS2)
Digital Devil Saga 1 (PS2)
And now, I'm playing Devil Survivor 2 (NDS)
There's a whole lot more and an anime for Persona 3 and 4. Seen both.
So, pretty much, somewhere in the story demons come into play. Demon doesn't literally mean demon, but anything occult/religious/cryptozoological. Also, I don't mean this as a generic statement as much as every game has an amazingly different story, game play and mechanics, graphics, and demons. In Devil Survivor you summon them with your cellular device, in Digital Devil Saga you become them and eat others, and in my favorite, the Persona series, they're a manifestation of your psyche represented by an actually named demon used to battle creatures born from human's negative emotions called Shadow. I've been wanting to run a Persona game forever.
Save for being RPGs, my favorite genre of video game, each has a unique, yet awesome battle system that really challenges players to use extreme strategy and tactics and I love them all!
Well...except Strange Journey. I like it as a friend.
Anyone else play these, or am I the black sheep?