In terms of reception including rural - Spark is the best. I have used 2degrees and Vodafone and found thier reception patchy.
Spark is very good in Data speeds - Especially 4G. 3G data speeds gets slower as most of the users are still on 3G for voice.
Spark has 4G calling - which makes it the best for data and voice.
Incorrect information in your post @jalvares VodafoneNZ has VoLTE live NZ wide as well
I agree on Vodafone's VoLTE NZ wide. I was just referring to Spark in my post.