Which is best field in mechanical?

There is no "better" or "worse" department / type of work / field. It all depends on what YOU can do, relate to and / or what value YOU can bring to the job. 

or example, if you are a mercurial person and you don't like repetitive work (I am one, for example), Production is not the field you should be in - it is a job which is repetitive day in and day out, and your work will be about keeping the throughput to the desired level.Quality Control and Assurance is about measurement and statistical analysis. Do you have a gift for this? Don't consider it boring - it has greater value add than all the other departments.Design is not done in most production organizations. You have to be part of an organization which designs products as it's primary operation, like designing and constructing custom production machines. Do you have the necessary skill, visualization abilities and - most importantly - courage to fail?