In my opinion...the most memorable evolutions in each series:

Season 1:Agumon and Gabumon into Wargreymon and MetalGarurumon for the first time


Season 2:Paildramon into Imperialdramon for the first time


Season 3: Guilmon into Dukemon (with Takato) for the first time



Season 4: Takuya and Kouji into KaiserGreymon and MagnaGarurumon for the first time



Season 5: Agumon, Gaomon and Lalamon into Shinegreymon, MirageGaogamon and Rosemon for the first time



Season 6: Shoutmon into Omegashoutmon for the first time


and If I count Hunters as a different season: Arrestardramon into Arrestardramon Superior Mode for the first time



All of them still give me chills while watching them