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is the most common word used on your index page the main keyword?
That'd sort of be going about it backwards. You should first determine what the best keywords and phrases are for the site, and then develop your content with those words in mind.
The most frequently used word on your index page as it is might be a good place to start, but the best keywords are generally those that strike a balance between actually being searched on and not being impossibly competitive. What that means depends a lot on how optimized the other sites might be, and how effective your own skills might be.