I was a huge fan of Euro metal in the 80's and still am to a certain extent.
1, Celtic Frost were arguably the greatest at the time
2, Rage blew me away and still follow them
3, Mercyful Fate/King Diamond is the king of hell
4, Gamma Ray are still amazing, they totally blow Helloween away and in all honesty it was Kai Hansen's musicianship that got them where they are the time.
5, Triptykon, Tom Gabriel Warriors new band is fucking lethal. If you get the chance check them out.
As for the big 4, Megadeth are the top of the heap for me. Killing is my business is so underated it beggars belief, i mean what a marriage that was. 4 diverse talents in 1 band, two metal heads in Dave Mustaine and David Ellefson, and two jazz players in Chris Poland and Gar Samuelson.