None of them.
Even perfectly blizzlike servers are using blizzlike PvE values that are with nowadays knowledge and gaming skills makes experience very far from Blizzard exp in actual time.
Vanilla servers are so much ez compared to 2006 - avg raid DPS is like twice higher than back to that days - so you can even SKIP and COMPLETELY IGNORE some boss mechanics and even phases(eg Ragnaros Submerge, Noth phases, dispell/decurse at most MC bosses, Vael is just 40-50 seconds nuke with only 1 tank, you're good with just 1-2 Hourglass Sands per player at Chromaggus, Phase 1 at Nef is really joke - you're nuking mobs few seconds before another one will spawn etc) also you WILL NEVER get close to boss Enrage Timers - its common to kill Patchwerk(DPS check boss back to 2006) in ~3min with 7min Enrage timer.
TBC goes same way as Vanilla, but here added huge part of gearing - when back to 2007 most players req pretty much time to get attuned/geared, in modern TBC you'll get this like in few weeks. Plus arena abuse...
WotLK servers are also full with donated gear and player pool isn't static - people joining and leaving very fast.