Which agency do you recommend for booking a tour to Chichen Itza in Cancun, México?

I will be traveling with my family to Mexico next week and we would like to visit Chichen Itza, one of the wonders of the world. We were advised not to go on our own and I see there are a lot of agencies. We were warned to be careful with the agency we choose since there were many that did not really provide the services they offer. ¿Could you recommend the best travel agency to book a tour to Chichen Itza in Cancun?