better branch at rank1800 at main iits

I would suggest you to go for CSE . You shall get it either at IIT BHU or roorkee or guwahati . Order is CSE roorkee > CSE BHU > CSE guwahati

You won''t get CS at BHU, Roorkee or Guwahati with that rank. Best option is either BHU Electronics or Electrical and then Guwahati Electronics if you want something related to CS. If you like Mech then blindly go for BHU Mech. It''s best and largest department of BHU. Roorkee Electrical is good option also but it may close just before your rank.

If you want to keep core and non-core option open then I think your priority should be,

BHU Electronics > Guwahati Electronics > Roorkee Electrical > BHU Electrical > Guwahati Electrical

You have lot of options at old IIT at your rank. Take it based on your interest.