Well to me, it doesn't matter which genre is the hardest to master ... what really matters is that when you wake up every morning .... what is the tune that rings in your head?
Well Jazz is a very complete genre of music i should say ... it is rhythmically, Melodically, Harmonically complex and sound. But ask a jazz musician to play you a rock song .... you probably won't get much conviction as much as you would get from a true blue rocker.
As for me, i hear rock tunes all the time. But everytime i pick up the guitar ... i play melodic patterns and chord tones ... i guess i just feel it everytime my fingers sink into the fingerboard.
But to really study how Jazz musicians think or feel .... it's a totally diffferent dimension altogether. You can no longer think or apply what you know from Rock, Pop and Metal. It's not a a matter of difficulty .... it's a matter of difference.
Well jazz is still a place that is beyond my reach. It's like a difference between 2D and 3D. There's a difference between playing a scale with warpspeed and pinpoint accuracy and being able to handle rhythm, chords and melody at the sametime.
Ultimately, just play to your heart ... To each Genre for itself.