I am a BCA Graduate . Am I eligible for the PCS Exam or not ?Which subject is better ?

Yes definitely being a BCA Graduate you are eligible for PCS Exam .
Here I am providing the information about subjects in Combined State Civil Services examination after passing BCA course..
Better subject after BCA are :

English
Economics
Management
Commerce

List of Optional Subjects (Literature) - CS (Main) -
Arabic
Gujarati
Sindhi
Bengali
Kannada
Oriya
Tamil
Chinese
Kashmiri
Pali
Telugu
Manipuri
Sanskrit
Assamese
Hindi
Nepali
English
Konkani
Punjabi
German
Malayalam
Russian
Persian
Urdu
French
Marathi

List of Optional Subjects - CS (Main) -
Agriculture
Management
Chemistry
Philosophy
Civil Engineering
Physics
Commerce & Accountancy
Political Science & International Relations
Economics
Psychology
Electrical Engineering
Animal Husbandry & Vetinary Science
Mathematics
Anthropology
Mechanical Engineering
Botany
Medical Science
Public Administration
Geography
Sociology
Geology
Zoology
Law
Statistics
Indian History