I'll start by saying that defining the best drummer ever or the best drummer in a genre is impossible, however I'm happy to join in an informed discussion and take on board people's opinions. Here are the two Metal moments that I remember 30 years on because of the impact of the drumming.
1. Listening to the opening of the Restless And Wild album by Accept. Fast As A Shark starts with a German folk song played on a scratchy record, the singer screams, the riff strikes up, THEN Stefan Kauffmann erupts with a double drum beat the likes of which I had never heard before. Please note, "the likes of which I had never heard before", I'm not saying this hadn't been done before, I'm not saying it hadn't been done faster or better before or since, I'm saying that I had never heard anything like it and in the context of that song at that time it made a huge mark on me. Stefan Kauffman isn't a candidate for greatest Metal drummer of all time, but for me and people like me at that point in time, his drumming and its placement front and centre in the song raised the bar.
2. In 1987 (iirc) someone loaned me a copy of Slayer's Reign In Blood album. I hadn't been exposed to thrash before, and all the points I've just made stand in this case too. But Slayer and Dave Lombardo changed everything, I maintain that in terms of brutal in your face Thrash, Reign In Blood hasn't been surpassed in nearly 30 years, so best drummer in Metal or not this counts for an awful lot.