Season 1 Enemies
Devimon was the Digidestined's main enemy in the beginning. He turned good Digimon evil with his black gears. He was too powerful for most of the champion digimon to defeat, so Patamon had to digivolve to Angemon to defeat him.
Etemon / MetalEtemon
Etemon made the Dark Network, with which he ruled over the continent of Server. He was an ultimate, so it took the strength of MetalGreymon to beat him. Later in the season, he came back as MetalEtemon and defeated SaberLeomon, a friend of the Digidestined, but Zudomon got revenge with his Vulcan Hammer.
Myotismon / VenomMyotismon
Myotismon was constantly trying to get into the real world and he finally did. He spread terror there until Wizardmon sacrificed himself to give Kari her crest, which allowed Gatomon (Myotismon's former servant turned Kari's partner) to digivolve into Angewomon. After that, he regenerated himself as VenomMyotismon and Agumon and Gabumon warp-digivolved for the first time ever to defeat him. But he was not finished yet, and came back in the second season.....
The Dark Masters - MetalSeadramon, Puppetmon, Machinedramon & Piedmon
When the Digidestined defeated VenomMyotismon and went back to the digital world, everything was in chaos. The Dark Masters caused it all. At first, they seemed invincible, but Izzy found that WarGreymon's claws were deadly against Dramon digimon. WarGreymon soon wiped out MetalSeadramon. Puppetmon was hard to beat because he could control Digimon, but MetalGarurumon eventually destroyed him. Then the digidestined ended up in Machinedramon's underground kingdom, and WarGreymon finished him off too. Finally, they went to Spiral Mountain, where the last Dark Master, Piedmon, lived. He turned everyone except TK and Kari into keychains, but Angemon finally evolved into his ultimate form, MagnaAngemon, and together with Metalgarurumon and Wargreymon, he defeated Piedmon.
The final enemy, Apocalymon, first attacked the Digidestined with his claws. However, the Digidestined's Digimon digivolved into their ultimate and mega forms and destroyed all of them. WarGreymon and MetalGarurumon almost finished him off, and then he self-destructed. The explosion would have wiped out the digital world and the Digidestined, but they used their digivices to form a force field to trap the explosion. Finally, the Digital World was saved.