Depends who you ask.
I saw all-world superstar in Mahomes the first game I watched of his at Texas Tech (I was screaming for the Panthers to Draft him). Obviously he had that ridiculous arm talent (and the ability to throw from multiple platforms), but it was the understanding of the offence that blew me away. He knew when to risk a throw, how to throw guys open, where guys were going to work towards if the play broke down.
Rosen I never saw what the big deal was - I felt he was inaccurate in college, which never bodes well for the NFL.
I didn't fancy Mariota coming out of college and I think Winston has turned out how you'd expect as a pro (albeit he's still young for a QB) - big play potential for both teams. If the Saints can refine him they've got a capable starter there.