Hi there, thanks for your query.
I may be a little bias as I live in Christchurch, and that would get my vote. The South Island is renown for it's scenery and mountains, with the Southern Alps being the so-called back-bone of New Zealand. There are mountains in and around Wellington too, but Christchurch is in closer proximity to more of them. As for Auckland, there are hills around the place, but it has nothing on Christchurch or Wellington.