There is nothing such thing like best and worse to get into PSU because PSU requires skillful personnel. Among mechanical or civil, both have almost same number of recruitment.
But in little bit difference, Civil has little more recruitment.
Hope this will help you.
If PSUs are to be concerned let me tell you their are more job opportunities for a Mechanical graduate than a Civil one. The reason being as simple as Mechanical as a branch has more outlets in the industry and there are immense scope for the same. Civil too has a great intake but it doesn't compare to that of Mechanical.
Good Luck !!