There is no right or wrong answer, as on their day, both can produce exceptional bowling.
Averages show one thing, their whole career.
Rankings show form over the past few years.
Stats, I think stat are great and you can pick out one small stat to make your whole argument....
For instance, around 74% of Anderson's wicket are top 7, over his whole career. This drops to around 70% for Mitchell Johnson.
Therefore, taking this one piece of information out of context of all the others, like so many aready have done on here, you would have to rate Anderson higher than Mitch, overall in career terms.