Don't worry about it! IMHO is still better to not have much variety as opposed to go berserk. By berserk I mean composing chaotically, throwing theme after theme, melody after melody... It's always easier to add something new than to take something out. At least this worked for me :D
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I would personally say System 42, which is my favourite.
But in general I found two things among the whole OST which made it feel little bit repetitive. First is that bassline stays on one tone most of the time in the songs. Unless it was a goal (which can be of course) I find it a little lacking - too stable to say. Second is that the sounds are more or less the same through the whole OST. For instance: you can induce more variety in one song by repeating the melody and harmony but giving the melody (or even harmony) to another instrument each time, thus making slight change in overall feel while not having to redesign the melody altogether.
But take my advices with a grain of salt, I am no music theory expert, nor teacher :)