Sorry for the audio in pieces (I've since fixed the issue)
TLDW
At lvl 33 with rested exp

  • Classic - Stockades - 11.7k HP - 302 exp
  • Cataclysm - Blackrock Caversn - 7k HP - 264 exp
  • TBC - Ramparts - 5k-6.6k HP - 398-456 exp
  • TBC - Mana Tombs - 5k HP - 398 exp
  • WotLK - Halls of Lightning - 6.4-6.9k HP - 380 exp
  • MoP - Mogushan Vaults - 5.3k HP - 264 exp
  • WoD - Iron Docks - 8.2k-8.8k HP - 264 exp
  • Legion - Darkheart Thicket - 7k HP - 264 exp

With a final tally, Burning Crusade is by far your best option after the level squish, being both the highest exp per kill, and the lowest hp per mob. This also includes bosses which have a jarring difference in HP from 43k on some TBC bosses vs 70-80k+ on different expansions.