Influential within the realm of d&d: AD&D, for all the awesome settings that still shape the game today. Dark Sun, non-vancian psi, Spelljammer, Planescape, FR, Dragonlance, dragonborn being knockoff draconians, etc.

Influential outside the d&d brand: 3.x, for the OGL.

Popular by numbers: 5e, because advertising.

Popular by the love & dedication of fans: tough one but probably BECMI, maybe 3.x or AD&D.