Hard to say just one ! They have plants worldwide.
As Honda is world recognized brand, it may have many branches, so it is difficult to tell you exact.
Honda Hamamatsu Factory - Hamamatsu, Japan
Honda Hosoe Plant - Hosoe, Japan
Honda Kikuchi Plant - Kumamoto Prefecture, Japan
Honda Japan - Suzuka, Mie, Japan
Honda Sayama Factory - Sayama, Saitama, Japan