After 1.d4, best defense to learn for the beginner?

The one and only way a beginner may understand and employ hypermodern openings succesfully, is cheating.

I agree that a beginner should be exposed to various openings in the form of annotated games with various openings, NOT theory on various openings.  However, those openings should be confined to classical openings.  Queen's Gambit (Accepted, Declined, Slav), Nimzo-Indian, etc (QP Openings where Black doesn't relinquish e4 so easily), Ruy Lopez, Petroff, Philidor, French, Caro-Kann, non-theoretical Sicilians (i.e. Taimanov, Kan, etc - Avoid Dragons and Najdorfs), and determine from that confined list what matches your style of play.

That said, the best way is possibly to develop your first opening like a baby develops being left- or right-handed.  Naturally.  I played a bunch of blitz games in college, not knowing anything other than the rules and having played occasional off-hand games for 12 years prior, and I played what "felt right" after trying a few things, and low and behold, I asked if what I was playing had a name, and the response?  "Yes, that's the French Defense".  There's how I became a French player for 10 years before moving on to what is more my style of play.

When I started learning openings as a strong beginner, I favored Indian defenses and the Sicilian. I won a lot of games, but my positional understanding was set back until I learned to play the Slav and Semi-Slav. I recommend that beginners play 1.e4 for 100 games, then 1.d4 for 100 more. From the Black side, both of these moves should be met with the same for 100 games. The number 100 is arbitrary and means "extensive experience."