I highly recommend Rainbow for preschool. My niece attended two years of preschool there (their program is 2 years in the same class, which is great) and is now starting kindergarden there. The Montessori method is fabulous for preschoolers to learn the basics in a very "hands on" environment with the children allowed to choose their activities during the day (a lot of independent activity). They expect a lot from the preschoolers, so it's best if they are pretty mature.
Not overly academic for preschoolers (I consider
Challenger too academic oriented for preschoolers)