For me it'll be Namba area. I usually stay in Hotel Dotonbori, rooms are small but the location is perfect and the rooms are pretty clean. Short walk to subway, JR stn, Nankai stn, Dotonbori, Shinsaibashi and it's located in Namba.
Lots of cheap hotels near Shin-Imamiya stn, especially good if you are on JR pass, as it's on the JR Osaka Loop line