50-59, 1 Level Per Zone

[Rygar]

50-59: The Hole (I love it that much)

Realistically I think they'd go:
50: Lower Guk
51: CoM
52: Sol B
53: Kedge Keep
54: The Hole
55: KC
56: Velks
57: Chardok
58: Sebilis
59: Howling Stones

Only meh zones are Velks and Chardok, but not sure if there are other good alternatives if doing only 1 zone per level. Depending on class you could work in Siren's Grotto (but very tough zone) or an outdoor zone like Western Wastes if you can charm / fear some animals.

Considered Kael but that is more for faction / drops not exp.