I have always felt Iron rank gives the best dollar-to-content ratio, followed by diamond only if you play Skywars or Eggwars frequently. Lapiz and Obsidian feel like a waste of money to me, they really don't offer anything that justifies the extra money. Emerald is the most visually satisfying imo, and opens up a lot of new options for MinerWare.
It does really depend which games you play, but I would rank them from best to worst as: Iron, Diamond, Emerald.
Course, if money is no question to you, Emerald looks really cool and flying in lobbies gives you attention from normies (;
I went from Iron > Lapiz > Gold > Diamond, personally, though this was way back in 2014, so it isn't really relevant to today.