We recommend to stay nearby the train station of the line C-4, because it is very convenient to get to the Campus.
Station Chamartin: Hotel Via Castellana (4*), Hotel Exe Plaza (4*).
Station Sol: too many options. It's the truly downtown of Madrid.
Station Atocha: AC Hotel Carlton (4*)
Chamartin is closer to Campus Cantoblanco than the others.
Also, there is a town nearby the Campus, Tres Cantos. We recommend https://www.trescantos-hotel.com The train to Tres Cantos is around 10 minutes.