Persona 4 Golden is one of the most difficult games to unlock the best ending without a guide. So here are some easy things you should NOT neglect.

1) Max out Marie's social link before the end of the game. This is required if you want the best ending.

2) Max out Adachi's social link. (This is not required for the best ending, but it does help out).

3) Dec 3 is a very important day where you will need to answer a series of questions. This is difficult and extremely specific. Not to mention, your emotions during the moment may cause you to not think clearly. Here is what you have to answer without spoilers.
. "Wait a second here..."
. We're missing something."
. "........ true feelings."
. "Something's been bothering me."
. "We're missing something..."
. "Calm the hell down!"

4) Once Step 3 has been completed, you'll be asked to name the culprit out of a list of suspects. Take your time, look over each name, and really think about it. Make an extra save just in case you mess up.

5) In January, talk to Margaret when greeting everyone a Happy New Year and go immediately with whatever she tells you to do.

6) At the VERY end of the game. When you have to say goodbye to each social link and the game asks "Do you want to go" after talking to each one. DO NOT LEAVE. Instead, go to Junes for a little scene that will continue the story. Go and talk to the people at Samegawa Riverbank. Then visit the Velvet Room. As long as you talk to a specific someone after that, you should be good