Haven't yet seen Legion, but I chose Wotlk, because of lore. The first time I did Fjord, Stormpeaks and Icecrown I was so amazed that no xpac before or after made me so astonished. WoD has the best experience in terms of smoothless gameplay and a bit more engaging overall, because of many cinematics.
Then I would pick Cata for gloriously looking zones, disliked for meh lore and epicness. MoP and BC were ok for me. (Of course MoP has all improvements).
Also Vanilla sucked balls, except for class quests. Revamped Cata was and is awesome. Better than MoP and BC but worse than WotLK.
However I puke when thinking about doing Northrend, Cata, Pandas and Outland again, because I have 12 level 100 toons on my current realm, and about 15 toons levels 85-100 on older realms. I absolutely detest any questing but revamped 1-60 which doesnt really make sense to do because of absurd speed of lvling.
Also, Loremaster here.