Don't use this:
Blissey @ Leftovers
Ability: Natural Cure
EV's: 4 HP / 252 Def / 252 SpD
Bold Nature
- Softboiled
- Wish / Heal Bell
- Seismic Toss
- Toxic
Why it's bad: Blissey's not
bad, per say, but it's simply outclassed by its pre-evolution, Chansey, because Blissey has several key issues that prevent it from excelling. While Blissey possesses superior base stats than Chansey, its inability to use Eviolite means it has an extremely poor Defense stat that can be taken advantage of by nearly every physical attacker. Thus, Blissey is unable to wall the many mixed attackers popular in OU and often finds itself overwhelmed in a tier filled with powerful and unpredictable attackers. Furthermore, Blissey's niche over Chansey in being able to use certain special attacks has been largely overshadowed by a tier where Pokemon such as Assault Vest Excadrill and Aegislash take pitiful damage from even a super effective Flamethrower.
Instead, use this:
Chansey @ Eviolite
Ability: Natural Cure
EV's: 4 HP / 252 Def / 252 SpD
Bold Nature
- Softboiled
- Wish / Heal Bell
- Seismic Toss
- Toxic
Why it's better: With Eviolite in hand, Chansey becomes far bulkier than Blissey could ever imagine in both stats. Although the lack of Leftovers may seem poor, Chansey more than makes up for it by having comparable physical bulk to fully invested 100/100 defenses while retaining the greatest special bulk in the game. This means that Chansey can wall many unboosted attackers, even many physical ones—an amazing feat and certainly a lifesaver for any defensively-oriented or Cleric-reliant team. Many dedicated wallbreakers, such as Kyurem-B, struggle to get past Chansey, a testament to her effectiveness against offensive teams. Chansey is still usable in Sandstorm and when afflicted by status—most attacks will do substantially less damage to Chansey as compared to Blissey even when factoring in the extra passive damage. And although Knock Off is more prevalent than it was last gen, with very few exceptions, the users of Knock Off are not Pokemon that Chansey should be staying in on anyway.
Conclusion: By virtue of its greatly increased defenses, Chansey should almost always be used over her older sister. Chansey still has at her disposal the vast majority of her sister's tools: Seismic Toss and Toxic; Natural Cure; and a variety of team support options including Wish, Heal Bell, and even Stealth Rock. In a metagame filled with extremely powerful attackers and game-winning setup strategies, no defensive Pokemon can afford to be easy fodder. Chansey, unlike Blissey, possesses the sheer defensive prowess and resiliency to make a significant contribution to any team in need of a great wall.