Ah Blissey and Chansey. For generations they have been the go to Standard for Sponge. And with the highest HP in the game and Base 135/105 SDEF respectively, attacking them on the Special front is exercise in futility for all but the strongest of Special Attackers. However, thanks to Eviolite, Chansey has stolen Blissey's crown. Chansey is so much more durable than Blissey with Eviolite, its not funny, except maybe in an ironic sense.
Why Chansey?
+Higher Physical and Special Bulk with Eviolite -the numbers don't lie, Chansey takes hits better than Blissey with Eviolite:
252 Atk Garchomp Outrage vs. 244 HP / 252+ Def Eviolite Chansey: 261-307 (37.1 - 43.7%) -- guaranteed 3HKO
252 Atk Garchomp Outrage vs. 252 HP / 252+ Def Blissey: 357-420 (50 - 58.8%) -- 74.2% chance to 2HKO after Leftovers recovery
252 SpA Choice Specs Hoopa-Unbound Focus Blast vs. 244 HP / 12 SpD Eviolite Chansey: 304-358 (43.3 - 50.9%) -- 3.9% chance to 2HKO
252 SpA Choice Specs Hoopa-Unbound Focus Blast vs. 252 HP / 4 SpD Blissey: 370-436 (51.8 - 61%) -- 94.9% chance to 2HKO after Leftovers
+Weaknesses are easier to play around -Yes, Knock Off is problematic. Yes, any item tomfoolery hobbles Chansey. However, such strategies tend to be rare [Trick, Switcheroo, Magician, Thief, etc.] or predictable [Knock Off].
+Reliability -Chansey has essentially one set, but it is a very good set.
And with Walls, being able to stall out that extra hit can be immense in regards to Toxic damage and or stalling.
The Classic Egg
Chansey
Item: Eviolite
Ability: Natural Cure
-Aromatherapy/Stealth Rock
-Softboiled/Wish
-Seismic Toss
-Toxic
Nature: Bold (+DEF, -ATK)
EVs: 252 HP/244 DEF/12 SDEF
Classic Clerical Chansey. Aromatherapy is great for curing your team of Status, while Stealth Rock starts the Chip Damage game. Seismic Toss is mandatory, preventing Chansey from being complete Taunt bait while doing reliable damage. Softboiled gives Chansey personal recover, while Wish allows Chansy to bring back a weakened teammate from the brink back to fully healthy. Toxic rounds out the set, being the standard walls Standby and putting all of Chansey's foes on a timer until knockout. Natural Cure is great for opposing Toxic users, while Eviolite is mandatory for giving Chansey its Edge
Why Blissey?
+Less Reliance on Item -Blissey can run basically any item it wishes, unlike Chansey who is shoehorned into Eviolite. This gives Blissey a bunch of options, including Shed Shell for guaranteed running away, Leftovers to mitigate chip damage like Sandstorm, Hail, etc, a power boosting item for the rare offensive set, etc.
+More flexible offense -Blissey's SATK, while a meager base 75, is actually viable, as opposed to Chansey's unuseable base 35. That means Blissey can turn away from Seismic Toss and look to other options, prevent it from being Gengar/Ghost with Substitute bait (though to be fair, unless it is cursing, Gengar can't really due anything to Chansey or Blissey either).
Ardent Egg
Blissey
Item: Leftovers
Ability: Natural Cure/Serene Grass
-Calm Mind
-Softboiled
-Substitute/Toxic
-Ice Beam/Thunderbolt/Flamethrower/Dazzling Gleam
Nature: Bold (+DEF, -ATK)
EVs: 252 HP/252 DEF/4 SDEF
Calm Mind Blissey. Calm Mind actually works well with Blissey, allowing it to setup to punch through bulkier threats, while countering any special sweeper not using Tail Glow who think they can boost and pass through Blissey (few Special Attackers can boost fast enough to brute force their way past Blissey)
I.E.
252+ SpA Porygon-Z Thunderbolt vs. 252 HP / 0 SpD Blissey: 86-102 (12 - 14.2%) -- possibly the worst move ever
+2 252+ SpA Porygon-Z Thunderbolt vs. +1 252 HP / 0 SpD Blissey: 114-135 (15.9 - 18.9%) -- possible 8HKO after Leftovers recovery
+4 252+ SpA Porygon-Z Thunderbolt vs. +2 252 HP / 0 SpD Blissey: 129-152 (18 - 21.2%) -- possible 7HKO after Leftovers recovery
Softboiled is for Recovery. Substitute is for protection while setting up (all Sound Based moves (except Sing) are special, so Blissey's Subs are not easily broken by Special Attacks), while Toxic enables Blissey to do damage while setting up. Attack move is up to personal preference: each of those moves have a good chance of finding something 4x weak to it that even Blissey can exploit. Alternatively, you could throw out Toxic/Substitute for a second offensive move.