Best time to visit Ranthambore national park totally depends on what you expect from the park. Summers is usually a great time to spot tigers as the natural water resources dry out and animals in the forest depend on manmade watering holes for fulfilling thrust. So watering holes become the sure shot places to have good wildlife spotting during summers. But as the heat increases, it becomes tough for the visitors as well to do safaris during that period as well. So generally during those months you will find wildlife photographers and wildlife enthusiasts mostly going to the park, who look for good pictures of wildlife.