Testaverde had a solid team when he was playing with Cleveland/Baltimore in the 90's and couldnt do much with them. Like Edwin says, most of his numbers are due to long tenure as a player. Montana had better QB mechanics, desicion making, among other things.
Montana's numbers may not look impressive compared to a Manning/Marino, but his accomplishments speak in much better volume then his stats. 8x Pro Bowl, 3x All-Pro, 2x Superbowl MVP, 2x Regular Season MVP, etc. There is problay no QB that can match up with Montana in terms of accomplishments/awards. Also if you want to look at stats, compare his touchdown total to interception total... Which is very impressive.