Since my sister recently graduated from Thapar, I think I can answer this. Only three hostels, hostels B, C, and H are available for first-year boys.
Out of these hostels, at hostel H there are some AC rooms. It depends on your JEE Mains whether you will get any of the AC rooms at hostel H. Having a good JEE mains score will be beneficial. However, since everyone prefers AC rooms nowadays, the AC rooms get allotted very fast.
Hostel H is also closest to the academics block. It is pretty good. The rooms are spacious. But it lacks interaction with hostels B and C. It also does not have a badminton court or a garden area.