The subway system is very good in Stockholm, so we would recommend anywhere on the red line (T14).
KTH has its own stop (Tekniska Hogskolan) and the venue is 2 minutes walk from the entrance. Most hotels will be located south of that around central station, only 3 stops away (around 6 minutes). Stations further south (Gamla Stan, Slussen, Mariatorget, Zinkensdam, Hornstull) will be about about 10-18 minutes direct train. Most hotels are around Central Station, however Gamla Stan, the old town, is the tourist center and also has a large concentration of hotels.
Below are some suggestions to give you an idea of rates & distance along with links to Hotels.com searches around each station:
Walking Distance
Hotels.com search
6 mins: Ett Hem
10 mins: Elite Hotel Arcadia
11 mins: Clas På Hörnet
Around Central Station (3 stops from KTH)
Hotels.com search
Radisson Blu Royal Viking Hotel
Scandic Continental
Hotel C Stockholm
Comfort Hotel Xpress Stockholm Central
Around Gamla Stan (4 stops from KTH)
Hotels.com search
Scandic Gamla Stan
Victory Hotel Stockholm
The Collector’s Hotels
Hostels
Birka Hostel
City Lodge Stockholm
Generator Hostels
STF Vandrarhem af Chapman & Skeppsholmen
IYHF Youth Hostel Zinkensdamm