Yes, they can be Stunned, otherwise, Smiters would be useless, and Zealers or Frenzy would rule Uber.
Smite Stun does normal damage and Stun to all, but War Cry affects Champions and Uniques at a reduced rate of 33%. Uber usually reduce things even further, but I don't think anyone has bothered to bring a Singing Barb to an Uber fight. Barbarians hold the longest Stun possible coming from their Stun attack.
Recant. Check chart. War Cry is reduced versus Uniques and Champions, but Smite is not. Or maybe it is, but due to Fanatcism, the cooldown on each attack is < one second. Killa's Smite is quicker than Vanwa's Frenzy speed. FCR would likely be harder to fix versus IAS.
Stun does a 4.5 second Stun at level 20. (Barb)
Smite does 4.4 second Stun at level 20. (Pally)
War Cry does a 4.8 second Stun at level 20. (Barb)
Werebear; Maul does a 3.4 second Stun at level 20. (Druid)
Werebear; Shockwave does a 13 second Stun at level 20. (Druid)
Twister does .4 second Stun at all levels. (Druid)
After research, as it seems, a Werebear is the best idea to bring to a fight.







