1976 is an underrated choice because, although he obviously didn't win, Ford went from being down something like 30 points to losing by 2. Though arguably that was more of a Carter collapse than a Ford comeback, especially since the infamous Eastern Europe comments more or less kept Ford from going over the top.
On that same point, 1968. Around the time of the DNC, Humphrey was polling in the low-mid 30s and Wallace was hovering around 20%. 2 months later, Humphrey wins over the bulk of undecideds + some Wallace supporters. Nixon's support barely budged during this time.