Which are the best engineering Diploma courses to pursue after 12th? Can I get into 2nd year B.Tech after the same?

Every course is good. I would suggest you opt for computer science as it is a good branch and has good scope. You can choose a core branch too.

Every course is good. I would suggest you go with the core branches. You can adopt for mechanical, civil or electrical and definitely you can take admission by using this Diploma in the 2nd year of B.Tech. Good luck.