One of the most important songs in your game should be the main theme, if you are going to spend weeks working on composing one song in your game it should be the main theme because The main theme is just that important. A good main theme should leave a a good first expression; the song should blow you away at the title screen and when it's done it should make you want to start the game. Another thing a main theme should do is that it should set the mood or feel of the game. The main theme should represent the game, meaning the feel of the game should match the feel of the main theme. To put it simply, if your game is epic your main theme better be epic. If you want a good example let me present Planescape Torment
Right when you start the game you go through all of the company logo's that we are all familiar with then we get to the intro cutscene which is great. Once we get to the title screen the song begins and it's like a match made in heaven. It starts out with a bell toll and then these voices come in chanting. A buildup starts; with the tone of it ascending and at the end it has a quick climax and then the song truly begins. It continues on with a slow and calm pace kinda like the beginning of the game where you making your way out of the morgue.
Suddenly out of nowhere this epic chanting comes in and takes over. The chanting is like a mixture of an epic and awe type of feeling; I believe this section is all about game world; walking out of the morgue and into Sigil; that feeling that you just entered a big world and you are just a small part of it. another possibility is that it could be about The Nameless One; mainly his actions, his relationships with the other characters or the mystery of his past.
Finally, the last section has a more aggressive feel to it. This most likely represents all the conflict in the game which can be as simple as the combat with enemies or the conflict The Nameless One has with himself. After that the song slowly and quietly ends leaving you to reflect.
You see, every section of that song is important to the game; each telling a part of the story. Each section is different but they all fit together like a puzzle. This is a great example of what a main theme can and should do. If your main theme can't do this; then why exactly is it your main theme in the first place? When you're composing the main theme, you have to have the entire game in mind to make sure that it gives off the same theme and emotions. If you do that people will remember it and if they remember the theme, there is a good chance they will remember the game associated with it.
Note: Once again I didn't upload the song it was actually done on this guys channel.