We used Thrifty, reasonably priced and also we were able to pick up at Christchurch and drop off at Dunedin. Check the small print, as some don't allow you to use gravel roads - we drove round a few round the Catlins and Otago peninsula. Also be aware of the huge excess charged by most of the NZ car hire companies - paying the extra insurance lets you reduce this to a reasonable excess or even none at all - it makes quite a difference when comparing prices. All worth checking out before considering which to choose.