Go there in this order of priority

Sri Lanka
Nepal (I’ve not been back since the major earthquakes.)
India
Bangladesh

India was interesting, and in many ways overwhelming, for the little that I saw of it; mainly Delhi, and Lucknow. I’m likely to go back if the opportunity presents.

Bangladesh is a ‘smaller’ version of India as you’d expect. It has the all the same drawbacks of its former sibling, and in many ways Is indistinguishable, culturally.

Both are a seething mass of humanity, much often compressed into areas and situations by an overwhelming poverty that is largely incomprehensible to most tourists.
The majority religion of Bangladesh just adds an extra layer of depravity.

I lived off and on for several years in Dhaka, due to a previous relationship. There are many stories by CEO members regarding the negative aspects of Cambodia. My personal experience of Bangladesh and dhaka makes Cambodia seem well adjusted in many ways.

Sri Lanka by contrast is much cleaner, both physically, and morally. It resembles in many ways it’s big neighbour, but somehow the people there maintain a greater level of self respect and dignity.
Likewise, Nepal.

India is a fantastic country and you could spend decades exploring the many diverse sites it has. The food is in general really good too. The men are a bit too friendly though, there is always some annoying guy trying to ask you stupid questions like "What is your service?' etc. I was traveling with women both times I went there but don't remember ever talking much to any women there, there is a very chauvinistic society similar to what you might expect in the middle east. South East Asia doesn't have that so much.