For me it's Diablo II. Granted I've played my fair share of D2 since launch, and also recently on a private server with a comrade from hexbear, but I still feel like years later the game didn't grab me as much as D1 did.
Granted I don't hate D2, but for a game that I keep coming back to, D1 takes the prize.
For me, every single armored core game after last raven. Like there is just a lot lost when the series becomes designed around a regular twin stick control setup over the "bad" control scheme that the game was designed around before 4/FA.
Like it just feels much more rewarding to play and beat, and every time I play 4/FA or 6, I almost always want to go back and play 3 and Silent Line again.
I haven't played formula front, and I don't remember when that released.