I forgot to mention the music from Achievement Unlocked 3. It's great and fits well with this nonsense game. At the beginning, I thought it would become tiring after a while. But, surprisingly, it didn't.
The games I remember having great soundtracks are Raze 1 and 2, Strike-Force Heroes, K.O.L.M 1 and 2, and Armor Mayhem. Fantasy Online has a really catchy but repetitive soundtrack.
Protector I-III got a brilliant soundtrack. I just don't get it why they removed half the tracks in Protector IV. Well, at least "Alpha Omega" is still there
The level music in Monsters TD is well made and FUNNY %-)
It is the only game I know where the music is a in the form of many sound clips which are blended together to create 4 different versions of the same idea.
My top 3 are:Strike Force Heroes, Raze(1 andd 2), and Bunny Flags(1 and 2). They all have this awesome and encouraging soundtrack, perfect for shooting and action.
Space is Key 2 has a simplistic soundtrack but it's one of my favorites! It's just one long continuous track played over and over but it's a great track! Much better than the one in the original Space is Key game!
For games on AG, I would have to say Eridani, not because of the music itself, but because of the context of the music. Most of the game does not actually have music (but it does have some of the best sound effects around); the only time you hear music in the game is right when and after you complete a sector, but you're so relieved to hear it that you learn to love it even though the track itself isn't anything more than a few chords on a beat.
Also, it's not an AG game, but I absolutely love the menu screen music in PacXon. I'm sure at least a few of the 67 million people who have played the game just click on it to hear the music. I can't be the only one who does that.