There is no 'last city' because you can find any of the Kanto Region cities in practically any order.
http://www.serebii.net/heartgoldsoulsilver/legends.shtml go here it will tell u everything oh but not about all the 28 unknown but u can get them in alph ruins There are 28. 26 are the letters of the...
groudon
Entei, Suicune, Raikou, Lugia, Ho-oh (Ho-oh is after National Dex in Soulsilver). Mewtwo, Articuno, Moltres, Zapdos, Groudon, Raquaza.
to get lugia you have to go to the wirl islands and to find ho-oh you have to go to the tower