I assume anyone who comes here will either have completed a reaction or studied one (computationally, in a course, for fun, etc.) recently, so why not share favorites? Include why you liked it best, if you feel like elaborating!
I'll go first.
Mine might be the reaction of an oxime (in dichloromethane) with an O-protected propargylic alcohol, using bleach and triethylamine. The oxime gets converted into a nitrile oxide by the bleach and base, and then undergoes a 3+2 cycloaddition with the alkyne to produce an isoxazole VERY cleanly. Work-up of this biphasic reaction consisted of partitioning the phases and concentrating the organics (after a weak acid wash and silica plug), resulting in publication-quality NMR data without further purification going into the next step! Very good for me as it is a key precursor that needs to be made in very large (for R&D) quantities.
It's not super fancy by any means, but I love cycloadditions, heterocycles, and reactions that are successful with minimal byproducts (don't we all?), lol.