When I'm introducing friends to ZDoom, I typically show them .wads that show them how far Doom as come. I introduced a friend to ZDoom by first showing him WWHC-Diaz, Action Doom 2, and KDiZD.
It's these kind of mods that I feel are the best. When you play them, you can see how far people have taken the engine and made wonderful things to play out of it. There is no definite "best wad ever", however, but I do recommend playing a large variety of wads and forming your own opinion over the matter.
In terms of "add-ons", it's possible to say ZDoom itself, simply because it supplies so much more fun to the Doom engine we all know and love.