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.
Renamon - 'Digimon Tamers'
Unlike most Digimon with human partners, Renamon sees their partner, Rika, as a means to get stronger, not as someone to bond with - and Rika feels the same. Nevertheless, the two eventually come to care about each other over the course of their battles together.
Gomamon - 'Digimon Adventure'
Gomamon is Joe's partner Digimon. They're a fun loving, cheerful seal-like creature who helps Joe see the brighter side of life. Once they digivolve into Gaogamon, they also pack quite a punch.
Gabumon - '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?
Calumon - 'Digimon Tamers'
This Digimon is almost unacceptably cute. Besides being adorable, they're also a huge help to the Tamers. They may not have their own human partner, but they help the group find important information, and play a key role in allowing other Digimon to reach new heights of evolution.