Hinduism is traditionally a polytheist religion with a wide variety of Gods. In contemporary times, I find that, Ram and Krishna have come to be perceived as the primary Hindu Gods, atleast in certain parts of India. This is reflected in worships, temples, also in festivals such as Diwali, Holi which are central to Hinduism. How did these 2 Gods emerge to be central deities in Hinduism?
Hinduism is traditionally a polytheist religion with a wide variety of gods. Wrong. Hinduism is often misunderstood that it is polytheistic. This is extremely inaccurate. We are polytheistic, monotheistic, and nontheistic. All of us who have souls and hearts (athma), no matter who we are, are a part of the embodiment of Bhagavan. Bhagavan is defined as the combination of all our Athma (souls). While praying to Bhagavan, you are praying to all living things. We have no god in the traditional sense, but we worship Bhagavan. Bhagavan has an equal good and bad side. When you come down to earth, you have good and bad. If you have almost all good, then you can attain Moksha, when you never come back down to earth. This means you will never have to deal with evil, and you will be happy. Now, since everyone is part of Bhagavan, everything is God. So we have over a trillion gods, since we count microbes and bacteria as well. So, we are polytheistic as well. But then, Bhagavan is just the combination of souls, and it is only based on the 4 principles (Karma, Dharma, Samsara, and Moksha), so we are non-theistic as well!
Major Gods are different parts of Bhagavan. So, although me and you are both "gods", one good quality in all of us could be categorized as a major god who Hindus worship.
Rama and Krishna are not major gods at all, they are avatars of the major god Vishnu who has 10 avatars. Shiva, Brahman and Vishnu are the 3 most major gods. However, before praying to any other gods, we pray to the lord Ganesha, the removal of troubles.
To answer your question, Rama and Krishna both have the most great festivals. Other great festivals include Navarathri, but that is dedicated to all female gods, not just one singular god. Other great festivals include Pongal, but that is dedicated to the harvest, not a god at all!
When talking about festivals, please note that I am talking about Tamil celebrations more than the other variations of Hinduism.
Try to check out the book "Sri Rudram" by Swami Dayananda Saraswati to learn more, or go to your local sage. If you want to, you can learn Sanskrit and read directly from the Vedas.
All in all, we are still polytheist, in the framework of being monotheist, which is in the framework of being nontheist. We are not, and we were never, duo-theist.
Hope this answers your question.