I really like Viterbi. It has applications all across computer vision and filtering and it takes full advantage of the magic of dynamic programming.
Amusingly, despite having done a ton of Computer Vision (I worked for an image recognition company for 4 years) I never used it for that purpose. The main practical use case I had was related to the use of Trellis modulation [2] for band-limited communication channels.
[1] https://blog.separateconcerns.com/2013-03-03-viterbi-algorit...