Regice

Regice holds a niche as the only special wall in ADV OU that has both a specially offensive slant and a way to remove opposing special walls through Explosion. Its powerful Ice + Electric coverage together with the threat of Thunder Wave reliably forces out fast, frail special threats like Zapdos and draws in a whole host of bulky Pokemon like Blissey, Snorlax, Metagross, and Tyranitar. As a result, Regice does not only possess the role of breaking special walls, but it also serves as a key pivot to other offensive threats like Choice Band Metagross and Moltres. A downside, however, of the variety of Regice checks is that Explosion cannot reliably target the opposing team's special wall; Metagross and Tyranitar can switch in to absorb Explosion or even any other move at a small cost.

Despite Regice's strengths, Snorlax finds its way into offensive teams much more frequently for several reasons. First, Snorlax's Self-Destruct is extremely threatening, for even a failed attempt at taking out a special threat can result in a check like Tyranitar or Metagross being put into KO range of a sweeper like Zapdos, never allowing them to lay another hit. Second, Snorlax has solid neutral coverage for all of its checks, including Focus Punch for Tyranitar, while Regice has to deal with its checks by employing the support of teammates or giving up its walling abilities with Explosion. Regice's Explosion too is unfortunately weak and requires chip damage for the KO or momentum to be maintained for the damage to stick. Third, Snorlax's ability Immunity prevents Toxic from wearing it down when it already has limited longevity due to sand and Spikes. Regice, however, hits Skarmory really hard, and it is more effective at preventing Spikes from accumulating to an uncomfortable level. In spite of these challenges, Regice's ability of luring in and KOing Blissey and Curse Snorlax with Explosion may well be good enough of a reason to use it instead of Snorlax as a special check.