The card I'll miss the most is Mandatory Upgrades and for many reasons.
First, it's such a cool, powerful effect and it requires such a specific deck to be able to play it. It's both a challenge and a reward and I love that.
Second, my first really good deck was based around Mandatory Upgrades. UpInc, I called it. (Some called it "Man up" but I dislike that term). It had Encryption Protocol, Red Herrings and Ash on it as well as some cheaper ICE and Tollbooths to rush the Mandatory. It worked surprisingly well and many years later the Germans actually went and revived it, which was super cool.
It was also my most read article for a very long time. It was one of the first "primers", it featured some cool cards and it got traction through BGG when it was the only place you could read about Netrunner. Loved that deck.
Your opponent: You call that jank? Now THIS (shows Kraken) is jank.