Physical Attackers.
A lot of physical attackers smash straight through all that HP it has, by targetting it's measly defense stat. Good examples are mostly fighting types like Conkeldurr (which can gain almost it's whole HP stat back in one Drain Punch) and Breloom.
Pokemon with Psyshock / Psystrike / Secret Sword.
A whole lot of Pokemon adopt Psyshock instead of Psychic solely to be able to counter Blissey well. For Example Latios and special variants of Jirachi.
And Keldeo-Resolute can use it's huge special attack stat and target the defense with Secret Sword, which makes it an excellent special counter.
In Ubers, Mewtwo utilizes Psystrike, the better Psyshock, to completely annihilate any Blissy that stands before it.
Damage Calcs:
OHKOs:
>252 Atk Pure Power Medicham High Jump Kick vs. 252 HP / 252+ Def Blissey: 938-1106 (131.3 - 154.9%) -- guaranteed OHKO
>252 SpA Choice Specs Keldeo Secret Sword vs. 252 HP / 252+ Def Blissey: 750-884 (105 - 123.8%) -- guaranteed OHKO
>252+ Atk Choice Band Huge Power Azumarill Superpower vs. 252 HP / 252+ Def Blissey: 864-1018 (121 - 142.5%) -- guaranteed OHKO
>252+ Atk Life Orb Lucario Close Combat vs. 252 HP / 252+ Def Blissey: 905-1063 (126.7 - 148.8%) -- guaranteed OHKO
2HKOs: (taking 4 HP investment, believe me that is way more common than full investment)
>252+ Atk Conkeldurr Drain Punch vs. 4 HP / 252+ Def Blissey: 516-608 (79.1 - 93.2%) -- guaranteed 2HKO after Leftovers recovery
>252+ Atk Technician Breloom Mach Punch vs. 4 HP / 252+ Def Blissey: 390-462 (59.8 - 70.8%) -- guaranteed 2HKO after Leftovers recovery
>252+ SpA Life Orb Latios Psyshock vs. 4 HP / 252+ Def Blissey: 339-399 (51.9 - 61.1%) -- 96.1% chance to 2HKO after Leftovers recovery
>252+ SpA Mewtwo Psystrike vs. 4 HP / 252+ Def Blissey: 369-435 (56.5 - 66.7%) -- guaranteed 2HKO after Leftovers recovery
>252 SpA Mega Mewtwo Y Psystrike vs. 4 HP / 252+ Def Blissey: 402-474 (61.6 - 72.6%) -- guaranteed 2HKO after Leftovers recovery
Hope I helped!