Ryan had the greatest arm of any pitcher. There is no disputing that, and he is the Strikeout king, but he is not and was not the greatest pitcher. He walked too many guys.
The greatest goes hands down to Pedro. I watched him and Clemens for many years, and that was my opinion backed up with no stats. The stats however bear it out. Take a look at every stat that is important for a pitcher. Look at Pedro's rank on the all-time list for the following stats:
1. Winning %
2. Lg. Adj. ERA
3. Hits/9 innings
4. Hits + walks/9 innings
5. k's/9inings
6. strikeout to walks ratio
After digesting his rank in those all-time categories, tell me who else compares. The answer is clearly that he is incomparable.