To a point I can totally agree with you!!!
Their 2 best albums (Hell Awaits, Reign in Blood) > any other album by the big 4
Megadeth are very close second, after all they didn't turn completely shit until they made Risk, although Youthanasia was nothing amazing compared to anything else by them
and their first 5 albums are immense, even the new stuffs isn't THAT bad!
Can't decide who I like least out of Thrax and Tallica though.... they have both been shit for a large number of years and only have a couple of albums I really like....