Nah, it's a fair question for fans of anything, not just metal.
Mine has always been Kai-era Helloween; they did no wrong with him in the band. Keepers 1 will always be my favorite album, and Keepers 2 and Walls of Jericho aren't far behind.
If I'm not allowed to disregard post-Kai Helloween and have to take the band's entire catalog into consideration, then I choose Blind Guardian. IMO they have one of the most amazingly consistent and awesome catalogs for any long-running band.
Riot- hard to find a bad album anywhere in their 30 year history regardless of the lineup involved.
Running Wild- also consistently great, although the post-Noise years are a big step down from their former glories.
Misfits- Even most of the post-Danzig material was killer. The Earth AD era is their only so-so moment
Savatage- they had a great run, and I like most of their catalog. Chris Oliva is one of my personal true guitar heroes alongside Reale and Hansen.
Pentagram (US)- I own more Pentagram material than any other band. In the Dedications section of my doctorate dissertation, the last line reads "And of course, Blue Cheer". The editor didn't catch it
Armoured Angel- their catalog is more limited, but I always thought they were killer. Just about the only band that I would consider being a completist collector of.
"I'm sorry Sam, we had real chemistry. But like a monkey on the sun, our love was too hot to live"