We used Pacz Tours to go to ATM and Tikal. The ATM tour was one of the coolest things I've ever done and our guide Francisco was very detailed and conscientious. Just an incredible experience. Tikal was great too, especially the ease of not having to deal with the border crossing. It's a bit of a whirlwind tour, and my biggest piece of advice is to bring a snack! You stop at a restaurant after the tour and we didn't eat until 3:30 PM. Having had breakfast at 6 AM, I was ravenous at around 1:30. Overall, though, the company had good service, great guides, and made the experiences easy.