>>>Megadeth is definitely not one of the 25 most important Metal bands of all time
I think they are. I would put Dave Mustaine in a list of top 25 metal frontmen...so there's my reasoning along with the quality of music and staying power they have had.
I don't think Megadeth should be there. Maybe if it was top 40. Metallica is already there, and of course Dave was a big part of Metallica becoming the significant band they were, so Megedeth (Dave) is already indirectly represented.