Short answer, Legion.
Not an easy question to answer. I would divide WoW into time periods.
First era, Vanilla & TBC
Basic WoW.
Second era, WotLK & Cata
This is when things started changing, multiple difficulties, LFR and more.
Third era, MoP & WoD
We got new, slightly more advanced systems, such as reforging and the mission table. And Legendary items for everyone.
Fourth era, Legion and onward.
This is the current era of WoW, and definitely what I would consider the start of "Modern" WoW.
Seriously advanced system, such as alternate skill trees etc, major secondary progression systems.