Since Blissey was released as an evolutionary form of Chansey, it has always taken the glory of being the greatest Special Wall in game. With a base HP of 255 and SpD of 135, it is one of the few Pokemons in the game that can take a neutral Kyogre's full-HP Water Spout.
The 5th generation, however, changed this once again, as an item called Pre-Evolution Stone made Chansey much more of a wall than its elder sister, Blissey.
It is important to compare the stats between Chansey and Blissey by their base stats:
Blissey: 255 / 10 / 10 / 75 / 135 / 55
Chansey @ Pre-Evolution Stone: 250 / 5 / 37 / 35 / 215 / 50
The values are calculated by zee-brah on post #5.
*Note: This is only a comparison of the stats using the Base Stats. The Pre-Evolution Stone actually make changes to the actual stat (1.5x the stat normally for Def/SpD). You can look at it like a free Cosmic Power.
The most important aspect of using Chansey is, of course, special walling, therefore, the 157 SpD triumphs Blissey's 135 by quite a stretch. As it still has certain downsides from Blissey, such as not being able to fully utilize its SpA and cannot carry Leftovers, Blissey still very much has its usefulnesses. Its ineptability to effectly wall Ghosts other than statuses in such a way Blissey can also gives some people a doubt, but I do not foresee Chansey ending up in the UU tier again with such powerful stats unless there is an item ban. However, it is not a matter for me to discuss this issue.
Possibly Chansey set:
Chansey @ Pre-Evolution Stone
Nature Cure | Bold / Calm
4 HP / 252 DEF / 252 SpD [There may be better spreads]
- Thunder Wave
- Toxic / Stealth Rock
- Seismic Toss
Summary: Rather than saying Chansey is superior than Blissey, we can actually just conclude that it can be used different and very effectively. Both Chansey and Blissey have their pros and cons, but in term of Special Defense, the younger fatty wins.