The deadliest of them all: Top 10 killer mobs on Hardcore inferno
D3DB compiled an interesting statistics listing the ten mobs that are most often the reason for the message above.
It would likely be to nobody's surprise that the arcane sentries of the arcane enchanted elites are on top of the list (which, ironically, is no monster by itself). Second come Act I's Scavengers, followed by Belial, the Lord of Lies.
2) Scavenger - 311 deaths
3) Belial, Lord of Lies - 308 deaths
4) Walking Corpse - 305 deaths
5) Skeleton - 190 deaths
6) Dark Cultist - 174 deaths
7) Risen - 146 deaths
8) Azmodan, Lord of Sin - 144 deaths
9) Dark Berserker - 130 deaths
10) Carrion Bat - 113 deaths
This data is sampled from 8,928 inferno deaths, singled out from 45,693 hardcore deaths from around 2 million player characters with Belial being the top killer if all hardcore deaths are included.