If we're going based on statistics, of course it's Reigns. In reality, and based on popularity and overall talent, it's Seth.
And statistics are subjective anyway.
I mean, sure Roman has 3 World Title Reigns and Seth only has two. However, Seth was champion for 220 days altogether and Reigns 118. So what's more impressive, the guy who held a belt for 220 days and only lost it because he was injured, or the guy who can't hold it and lost it 3 times while barely having more than half the amount of days of Rollins' first win?
Seth Rollins is the only person to ever hold the WWE and US titles at the same time.
Seth Rollins was the first ever NXT Champion and he held the title for 133 days. Roman Reigns never held it or any title in NXT.
Roman Reigns was 2014 Superstar of the year.... Seth was 2015 Superstar of the Year. Plus a total of 8 other Slammy Awards compared to Roman's 6.
Seth Rollins won the 2014 Money in the Bank Ladder Match.... Reigns...well, you know...
Reigns was a one time tag champion...with Seth Rollins...
Statistically Seth Rollins holds a victory over Brock Lesnar... which since Lesnar's return only John Cena, Goldberg. HHH and the Undertaker can also make that claim.
Was able to rise to the rank of WWE World Champion without having to be the member of a famous wrestling family
Fans like Seth Rollins... oh sorry, that was a cheap shot
Honestly, you can find accolades and skew them in any way that makes someone look much better. And then you have to decide what sort of statistics mean more to you. Is it more impressive that Roman has 3 World Titles or that Rollins held his for nearly twice as long? It's all as subjective as simply liking the person or not.