Vanilla: Either Blackrock Depths or Blackrock Spire would be the go to answers in terms of size and epicness, but they might actually be TOO big.
BC: Magister's Terrace
WLK: Utgarde Keep/Utgarde Pinnacle. I would have them combined for a 7 encounter raid.
Cata: Halls of Origination really does make the most sense in this xpac.
MoP: Either Shado-Pan Monastery or Temple of the Jade Serpent. Always feels wonky to kill a Sha in a 5 man.