Lol I love both those bands! With Megadeth I find I go back to them much more than say, Metallica. Their music is more complex and just has more to chew on.
My favourite Annihilator album by far is Never, Neverland. It was the first one I heard and I love every song. I also like Pharr's vocals. Their later albums are pretty cheesy though, especially the lyrics. Jeff's such an awesome guitar player though, and I always wished he would hook up with a better 'metal' songwriter/lyricist.
Bands I could never get into: Exodus and Testament. Just boring songs except maybe a couple from each. Also Anthrax aren't great with some notable exceptions from their earlier stuff.
Slayer is obviously awesome, but Hanneman's songs were always way better than King's. And their guitar solos blow.
I have a soft spot for Overkill, but I find most of their stuff is filler.
Song writing is king, and Metallica/Megadeth are the best at that.