Great question and it was a tough one.
For this exercise, I have put together lists of batsmen, bowlers, keepers and all rounders. I then rank them across their play areas and then across one another.
Those rankings are based on skill set, importance to the team, performances vs contemporaries, longevity and so on.
Originally, I ranked Steyn higher in my list of fast bowlers but the more I delved deeper, the greater Anderson's body of work. He has more wickets in many weaker sides whereas Steyn played in the greatest ever saffer teams of the 00s and early 2010s.
Having said that, considering I myself once rated Steyn higher, it goes to show a certain element of subjectivity and lists can change over time. But I'm comfortable as is now, but there really isn't much in it. Each of the 10 sections is a give and take between the best players. Remember these are all great players, so it's about deciding who is THAT much better.