This list is rife with inconsistency. I can understand Young, Clemens, and Ryan for their staggering career numbers, but Koufax and Feller don't fit in if that is the criteria. Walter Johnson, Christy Mathewson, Warren Spahn, and Greg Maddux would be better fits.
Now, if we are talking about the 5 best pitchers when they were at the top of their game in terms of dominance, then this is who I want:
1. Pedro, Pedro, Pedro, always Pedro
2. Greg Maddux - only one pitcher has two of the top-5 all-time ERA+ years, and surprisingly it's him
3. Walter Johnson
4. Roger Clemens
5. Sandy Koufax
Close but no cigar: Randy Johnson, Lefty Grove, Bob Gibson