NRL
2013 - The SBW GF.
2014 - Souffs fairy tale
2015 - Cowboys first title [broke 20 year drought] vs Big Brother Brisbane
2016 - Sharks first title [broke 50 season/49 year drought] vs Big Bad Storm
2017 - Cowboys (coulda been a fairytale, but clawed their way to GF) vs Big Bad Storm who dominated all 2017 and was a perfect example of having first past the post win the shield in 2017.
AFL
2013 - Hawks vs Freo [Note - Hawks lost 2012 too vs Sydney]
2014 - Hawks vs Sydney
2015 - Hawks vs West Coast [Note - people starting to tire of the Hawks dominance]
2016 - Bulldogs fairytale - broke 62 year premiership drought vs Sydney
2017 - Tigers fairytale - broke 37 year premiership drought vs Sydney
So on the balance of GFs...the NRL had 3 fairytale type GFs in a row (2014-16), sharing the 3rd with 2 AFL fairytale type GFs (2016-17) (and both long suffering Melbourne teams).
And despite the AFL's supposed expansion being better than NRLs, their Sydney and 2 Perth teams didn't drag in bigger numbers for their GF, and someone can confirm if 2017's AFL GF brought in big SA / Adelaide numbers. Also, NRL is starting to get great numbers in Vic and good numbers in SA, WA, NT and something to build and - ahem - expand on.
And let's face it, AFL numbers are spread over 18 teams nationally whereas NRL it's over 16 teams in 3 states and 1 territory, and NZ (and have we included NZ figures for GF in any of the ratings since we actually do have a team there?!). And finally, the ratings have swung on options in regional areas due to WIN.
However improbable, my money is on Melbourne vs Brisbane AFL GF in 2018 and a Parramatta vs Warriors/Cowboys NRL GF in 2018.