Using the metric HP x (Def + SpD), the champion is Assault Vest Blissey with 152 HP EVs, 104 in Def, 252 in SpD, and a Calm, Careful, or Sassy nature, which scores 474,721. However, this does leave the physical side rather exposed; you're probably better off maxing out Def EVs at the expense of HP to better shore up the stray physical hits that may come its way; that configuration scores only slightly worse with 473,352. Assault Vest Blissey isn't actually the greatest idea, since it means you have no access to Softboiled (among other things), but that just shows you shouldn't be trusting one metric as the end-all be-all.
Eviolite Chansey: 430,782
Assault Vest Cresselia: 385,392
Assault Vest Shuckle: 345,992
Eviolite Type: Null: 321,110
Eviolite Rhydon: 305,948
Itemless Arceus: 288,156
Itemless Aegislash-Shield: 251,100