This little aircraft started the whole business jet industry back in the early 1960's. In fact, when Learjets first appeared there were certain performance figures that bested some of the military's front-line equipment!
While the latest models are certified to FL 410 and have grown far beyond the capabilities of the 23/24 Learjets, I've always had a soft spot for the originals. It has "the look" and makes a lot of noise for its size. (That's its downfall in this age of quiet turbines--the aircraft will actually be unable to operate in the US after 2015 due to its inability to meet EPA noise standards.)
The February 2014 LOOTM
Lear Model 23/24