For your camera body, like others have said the best lenses out there are the "L" series, but they are pretty pricey.
I've now decided to rent one for my next trip to Laguna Seca. If I'm getting the results I want this way, I'll just keep renting for the time I require a superior lens.
If I visit Laguna Seca maybe 3 times a year, it rental costs would allow for about 15 years of use before I had spent the same as buying the lens. Just seems to me like a sensible route to take.
Something to consider, even a great way of trying out a lens before purchase too.
http://www.borrowlenses.com/
Cheers
Matt