Persona 4 Golden has become my favorite game of all time, no joke. What makes it so good?
Well there isn't much I can say that hasn't already been said. But it's basically a game that has the full package. The gameplay (combat system) is intuitive and fun. Some people complained about the dungeon design, but I think they are very accessible and grinding in the game is actually enjoyable. So I personally loved breezing through the dungeons (especially how each dungeon was tied to a new member in the group, and had a deep personal/meaning to them). Plus, I'll take the unique dungeons in Persona 4 any day to the never ending dungeon that was Tartarus in Persona 3 (that shit became a mind numbing chore).
And what ties all of it together is the characters. They are all likable, and really fleshed out. By the end of the game, you feel connected to them. The game does a good job connecting these characters to the journey you all share, so by the end you aren't just the "hero" that defeated everything. It truly feels like a group effort. The social/relationship stuff adds to that, as the characters don't just grow physically, but emotionally.
Plus, I just loved the world/story. It has superb pacing, and is a good balance of funny/charming and mystery. So for me, it's just the perfect game that really brings all these elements together into one thing.