Halls of Valhalla by Judas Priest
Never heard this one before it came up in Rock Band 4, but thoroughly enjoyed playing it despite having to cut it down to Hard difficulty.
Number of the Beast by Iron Maiden
Number of the Beast was one of the first metal songs I ever heard, and is still one of my favorites to this day.
On a less metal note:
Milwaukee by The Both
Like Judas Priest, this one came up on a RB4 show and I quickly fell in love with it, largely due to the fun guitar part, but it's still a really good song on its own.
Finally, if anyone didn't hear, Foo Fighters released a free EP about 2 weeks back, Saint Cecilia (Autoplay Warning). I haven't listened to it a ton, but when I did I rather liked it compared to Sonic Highways, which I really couldn't get into like I started to with St. Cecilia.