If you are trying to rent at MCI (the airport), then all of the agencies will be affected by the 9% airport fee. "City locations" (those not anywhere near the airport) are not affected by the full tax load, but Kansas City doesn't have a lot of them, except for Hertz and
Enterprise. You would also have to pay a pretty hefty cab fare to get to any of those locations.
If you are asking about flying into another city to avoid the tax, it isn't realistic -- there are no other large airports within a five-hour drive. (The closest would probably be Columbia, MO, which is about 2.5 hours away. The only flights into it are via Kansas City and St. Louis, on USAir.)
You might want to try Hotwire or Priceline for a rental car at MCI; that will probably be your best bet on rates.
BTW: If you are interested in how the taxes break down: $4-a-day charge to pay for a downtown sports arena, a $5-a-day charge for construction of a consolidated rental car facility, an "airport access fee" of about $12 and state and local sales taxes totaling about $14. USAToday ran an article about 6 mos ago that showed that MCI has the highest rental car prices in the nation because of the tax rate.