There is no debate. Mike Mussina belongs in the Hall of Fame.
Start with the fact that his HOF standards are 54, as compared to 50 for the average HOFer. His HOF Monitor is 121, with a likely HOFer being over 100. His gray ink is 244, 23rd highest all-time for pitchers, with an average HOFer at 185.
He is 33rd all-time in wins.
Among pitchers with at least 200 career wins, he has the 12th-best winning percentage.
He has the 15th-most wins in baseball over the last 50 years.
Of the 68 pitchers to throw at least 3500 innings since 1876 (not 1976) he's got the best K/BB ratio of all. And he has the 13th-best K/BB ratio of all-time for ALL pitchers.
He has certainly benefited from playing on good teams, but not as much as you'd think. His actual W/L record was 270-153 (.638) and his neutralized W/L record was 247/161 (.605.) He'd still have won more than 60% of his decisions. His actual career ERA was 3.68 but thanks to playing in some good parks for hitters, his neutralized ERA was 3.25.
He has won 7 Gold Gloves.
He pitched in 16 post-season series, for a total of 139.2 IP with a 3.42 ERA. In two World Series, he pitched 18 innings with a 3.00 ERA, 5 walks and 23 strikeouts.
For pitchers active in 2008, he has the 3rd-most career IP for a pitcher with a WHIP under 1.200. The only guys ahead of him are future first-ballot HOFers Randy Johnson and Greg Maddux.