Take a good look at my
Sp. Def tiers. Blissey's score on this scale is 128 with 0 EV investment. The next best pokemon is Chansey, with 125 with 0 EVs. After that is Regice, with weaknesses to fire, fighting, rock and steel. (In comparison, Blissey has weakness to fighting and immunity to ghost).
Regice scores 123 with 0EVs on this scale, and 127 with 252 HP / 252 Sp. Def EVs, 128 with max EVs and +Nature.
After that, the next best pokemon are Uber Wall pokemon, Giratina, Lugia and Ho Oh.
Snorelax and Cresselia come after them.
It should be noted that none of these pokemon, aside from Blissey or Chansey (the #1 and #2 spot respectively) have a reliable recovery all the time. Cresselia comes closest, but she only gets 25% health back in a sandstorm IIRC.
In terms of pure statistical Special walling, Blissey is the top pokemon. Now, it should be noted that rock pokemon get a 50% boost to their Sp. Def in a sandstorm, but 252/252 Shuckle in a Sandstorm is only _slightly_ better than a 0/0 Blissey, while having a weakness to rock and water.
Of course, Blissey is a fat whore who can take hits, but she can't really deliver hits back. Milotic has a huge Sp. Atk and the nice resistances that come with being a water, and can take the occasional non-STAB T-Bolt. Regice and Snorelax can also actually threaten the opponent as well.
Cresselia is a more well rounded whore >_> She either has very high physical defense, or high Sp. Def. Your call, EVs matter more than her stats really, they're too close.
But yeah, Blissey does what Blissey does best, absorb Sp. Atks and then soft boil it off. And most of the time she doesn't even need too much EV investment, which is why the standard Bliss tends to be max physical defense.