I like the hamburgers at Kama'aina, a Hawaiian burger place. A bit hard to describe how to find it, it's near Shibuya. Leave JR Shibuya station at Hachiko exit, go to the Hachiko police box (koban), turn right, pass under the railroad bridge and walk up the hill for about 10 minutes and turn left at a large 5 way intersection. Kama'aina is a block away.

There's another branch in Yokohama.