This is a hard one to answer but I'll make you understand a few things about Nigeria and when it comes to food.
I am from Nigeria and first I'll want you to understand that Nigeria is a multicultural country. There are so many cultures in Nigeria that in a small state like Akwa-Ibom in the south or Nasarawa in the north, there are at least 10 tribes in each of those states. Now compare this figure to larger states like Kogi and Benue.
One with different tribes is that with different tribes comes difference in cultural practices and thus same with food preference. So the favorite food in Nigeria depends on who you ask, the answers will definitely be different with each person.
As someone who has travelled to many states in Nigeria and have tasted all kinds of food offered by the different tribes, I can confidently say that whatever answer you will get, majority will be in the form of soups and some form of swallow.
Common food with high demand in Nigeria include Afang soup, Vegetable soup of use Calabar people of southern Nigeria. Ewedu soup with Amala of the Yoruba people of southern western Nigeria, Oha soup of the Igbo people of south eastern Nigeria and when I lived in northern Nigeria, I think anything from Kuka to Krakashi that goes with tuwo is acceptable.
Unfortunately I can't upload photos using Musing, but I'll edit the post to add a few photos for you. If you planning to visit, you are welcome anytime.
For me, the best food, or rather, my best food is fried rice and chicken with salad and maybe some plantain.