Digimon is an anime full of awe-inspiring and beloved creatures, each with unique abilities and personality. To narrow down the best, it's a question of personal preference. Some favor adorable Digimon, like Calumon and Patamon, while others gravitate towards the more powerful creatures, like Lucemon and Devimon. No matter what they look like, the greatest Digimon add something meaningful to the story and stand out as individual characters, not just generic members of a species.
Following this logic, and emphasizing individual Digimon over their respective species, Gabumon's evolutions won't come in to play here. Gabumon refers to every form Matt Ishida's partner has ever taken, from Tsunomon to WereGarurumon and beyond. The only exceptions are fusions, since you can't omit Omnimon on a list of best Digimon, and because fusions create a new stream of consciousness.
Omnimon is a fusion of WarGreymon and MetalGarurumon, who possesses physical attributes and attack power from both. This includes using the Garuru Canon, a wolf-shaped ice blaster, and the Grey Sword, a nearly invincible piece of fighting equipment.
Much like Pikachu is the face of Pokémon, Agumon is the face of Digimon. They're a spunky, energetic, and compassionate partner. Though their signature attack, Pepper Breath, is pretty cool, they gets a lot more badass when they evolve into Greymon, MetalGreymon, and beyond.
Arukenimon created BlackWarGreymon in order to destroy the DigiDestined - but they weren't able to act mindlessly. Instead, they questioned his orders, as well as the purpose of their actions and their very existence. Their self-awareness makes them one of the more memorable characters in the second season of Digimon Adventure.
Gabumon is Matt's partner, and their kindness and empathy helps the prickly kid come out of his shell. What's more, Gabumon has some of the coolest evolutions in the Digimon universe - who wouldn't want a partner who transforms into a giant, metal wolf?