Having played a couple more games (I think I'm on 18 wins now) I actually changed my opinion. The matchup is certainly much more close than I initially thought. And I might actually slightly favor Ragnaros. Nefarian does have his crazy and unstoppable starts every now and again, but in terms of consistency Ragnaros does very well, with some very efficient clears/comebacks.
everyone that is voting Nefarian just sucks at the game, if you break it down they are very equal. I'm 100% win rate on both heroes, with 6 on Nefarian and 4 on Rag. People are going to cry and cry until Blizzard takes the time to ruin the game for people that are not trash...
I think with evenly skilled players Nef will win more often than Rag. Rag requires more luck than Nef too imo. Sure you've got nef's hero power, but I believe his hero power has the "Unstable Portal effect" where you have a chance to just outright win the game with it. I've won games where the hero power gave me Totemic Mights and Deadly Poisons, and I've also easily won games when the hero power gave me something crazy like Siphon Soul or something really good like Flamestrike or Polymorph. I also think Nef is more fun to play as compared to Rag.
Now I'm not saying it's something crazy like 80/20, but I'm saying that Nef is definitely better even if it is by a little bit.