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The monsters look kind of like mishmashes of lots of stuff which not everyone likes. Personally it used to be a big turn off for me before I could appreciate digimon designs. In comparison, Pokemon look a lot simper visually, with not as many details.
Also there wasn't really a good "digidex" like Pokemon has, which made it kind of difficult to keep track of digimon and their weird digivolution paths. Also the games don't really have any consistency between them, unlike Pokemon.
Anyway my favorite is Mastemon