And for completeness, here are the top bands by average rating across all releases, counting only the bands that have reviews from at least 10 different reviewers.  

#	Artist	Reviewers	Average	Spread
1	Repulsion	11	96.43	3.064
2	Esoteric (UK)	13	96.2	3.544
3	Gorguts	15	95.43	4.03
4	Lykathea Aflame	12	94.6	4.363
5	Atheist	12	94.56	4.272
6	Solitude Aeturnus	10	93.9	5.485
7	Sacramentum	11	93.7	6.067
8	Disembowelment	10	93.43	3.959
9	Deeds of Flesh	12	93.33	5.375
10	The Axis of Perdition	10	93.33	4.607
11	Martyr (Can)	11	93.29	4.399
12	Cult of Luna	10	92.56	6.735
13	Persuader	11	92.4	5.886
14	Katharsis (Ger)	10	92.0	5.715
15	Novembre	12	91.86	5.436
16	Vintersorg	21	91.67	6.968
17	Demilich	15	91.58	7.522
18	Saint Vitus	13	91.36	6.526
19	Belphegor (Aut)	15	90.94	6.571
20	Manticora	11	90.64	7.889
21	Windir	21	90.64	6.986
22	Agent Steel	15	90.2	6.002
23	Negurã Bunget	10	90.17	10.123
24	Pentagram (US)	10	90.13	6.827
25	Maudlin of the Well	11	90.08	8.558
26	Deströyer 666	15	90.07	7.676

 

Vintersorg and Windir are the only bands to get an average above 90 with 20 or more reviewers. So clearly those are the greatest bands in all of heavy metal.  

The worst metal band in the world is Apocalypse, who got an average review of 7.0 from 14 reviewers. Dishonorable mention to Six Feet Under, the only band with at least 4 releases and 10 reviewers who averaged below 50 (49.91 from 47 reviewers).