There is no such thing as BEST field!!
If you ask me I would say that research in every field, from Artificial Intelligence to Bioinformatics, is important. Every field has its own set of problems to be solved.
For research all that matters is your interest. You need to be obsessed with the problem you are working on and you need to be willing to do what ever it takes to solve the problem.
If you are not aware of different fields in computer science, I suggest you to look into this wiki link.