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: