A colleague of mine is running a Diels-Alder reaction this morning, and turned out to have never run one before, despite many years of experience in chemistry. (I'd bet, though, that a fair number of chemists who have run the reaction did it in an undergraduate lab and never have since). I've run them - although it's been a while - and I've done the Claisen rearrangement (ditto), the Knoevenagel condensation, the Barbier reaction, and the Henry reaction. I've done plenty of Horner-Emmons-Wadsworth reactions (although not in the last few years), Jones oxidation, Birch reduction, the Arbuzov reaction, and a Chichibabin pyridine synthesis, many years ago. And I've done a Cannizzaro, the Gabriel synthesis, Ferrier rearrangements, the Shapiro reaction, Peterson olefination, and Lindlar reduction. I've run Sandmeyer reactions, the Prins, Staudinger reduction, Ullmann coupling, and Weinreb ketone synthesis. I've done the Wolff–Kishner reduction (once) and Wurtz coupling (once), a Dakin-West (once), a Darzens (once), and a Delepine reaction (once).
But I've never done a straight aldol condensation, at least, not on purpose. And I've never, as far as I can recall, actually done a Fischer indole synthesis, or the lovely Skraup reaction. I've never run a Bayliss-Hillman, a Ritter reaction, a Cope rearrangement, a Julia olefination, a Pictet-Spengler, a Nazarov cyclization, nor a pinacol, and I don't think I've ever set up an ene reaction.
So what's on your list? What's the most famous reaction you've never run? Is there some reaction you've always sort of wanted to do, but never had the reason?