I've gone over my general starting pitcher considerations here.
You can make a strong case for Clemens as the greatest pitcher of all time. Remarkably Durable across 24 seasons, Clemens replaces Maddux's long, gracious career arc with 3 distint peaks, centered around late 80s with the Red Sox, his two years with the Blue Jays in the late 90s and a smaller one with the Astros around 2004. Powerful, with overwhelming stuff, he was hard to square up on, with only a 0.66 HR/9 rate. His fly ball tendencies, high strike out rate (8.55 K/9, 23.1%), ruggedness and somewhat lesser control (both with the ball (2.89 BB/9) and with his bat hurling temper) remind me of, well, Curt Schilling on steroids. It's worth noting that the considerable majority of Clemen's innings were in the American League, so that 8.55 K/9 is somewhat depressed compared to Schilling's numbers.
Still, an amazing athlete and pretty much the definition of power pitcher, Clemens commands a spot on this team. His career line of 3.12/3.09/70/70 (ERA/FIP/ERA-/FIP-) with 145.5 fWAR is overwhelming. His best season is a hard choice, but I'll take 1997 with the Blue Jays: 2.05/2.25/45/50 for 11.1 fWAR. Roger Clemens, the Rocket, too close to the edge for comfort but damn impressive to watch.
There are two Tom Seavers. The demigod that pitched for the Mets and mere human baseball star that pitched for everybody else. It's the demigod that earns the mention here, particularly for his 1.76/1.93/51/59 line in 1971, with a K/9 rate of 9.08 and a BB/9 rate of 1.92. The human's tenure brought that down to 2.86/3.04/79/85 for his career, but it's still enough to rate Seaver among the greatest pitchers of all time.
Ryan, on the other hand, managed to remain inhumanly overpowering for over 25 years. He just had issues with that, you know, strike zone thing. He got better at it, though, going from a BB/9 rate of 6.87 with the mets in 1970 (his worse year) to a BB/9 of less than 4 for most of the 80s and early 90s. Still high for anyone else, Ryan was so hard to hit (lowest batting average against of all time at .200, a K% of 25.3%, and a K/9 of 9.55), he still managed to be a great pitcher without the control. His lifetime line of 3.19/2.97/90/84 suggests that he was maybe somewhat better than his ERA would indicate, but it's not quite enough to get him on this list. If you can only take pitchers over 40, though, he's a definite pick up. His best season was probably in 1987, 2.76/2.47/69/62 with a 11.48 K/9 (that's a 30% K rate for a starting pitcher). The Ryan Express was 40 years old.