Since people don't believe me, I will explain.
Nolan Ryan had an amazing career. But he did only 2 things better than anyone ever has: Throw really, really hard for a really, really long time.
Because Ryan played on some cruddy teams, we'll ignore Wins and Losses.
Let's look at strikeouts:
Seaver: 3,640
Ryan: 5,714
Pretty obvious who wins. Ryan's 5,714 were a result of throwing hard for a long time.
Let's examine there control by looking at K/BB
Seaver: 2.61
Ryan: 2.04
Seaver wins. Ryan had horrible control early on. In 5 of his 6 300 K seasons, Ryan never had fewer than 157 BB. The one season he didn't he had 301 K's and 98 BBs in 1989.
Let's examine WHIP
Seaver: 1.121
Ryan: 1.247
Pretty big difference
Now H/9
Seaver: 7.47
Ryan: 6.56
Ryan wins. Another effect of throwing very hard.
BB/9
Seaver: 2.61
Ryan: 4.67
HUGE difference here
K/9
Seaver: 6.85
Ryan: 9.54
Another huge difference, this time favoring Ryan.
ERA
Seaver: 2.86
Ryan: 3.19
A pretty big difference
Total
Seaver: 4
Ryan: 3
Finally, Seaver won 3 Cy Young Awards and finished top 5 eight times, 2 other times he finished 7th and 8th. He was a ROTY and finished top 10 MVP five times.
Ryan never won a Cy Young Award. He finished top 5 six times, 2 other times he finished 8th and 9th. The highest ranking he reached in MVP balloting was 14th out of 6 times.
In conclusion, Tom Seaver is better.