Seasons 1 and 3 are perhaps the greatest children's anime ever made
This review will only be of Digimon Adventure (season 1) and Digimon Tamers (season 3).
First off, Digimon Adventure. I know, I know, it looks like a Pokemon ripoff, but trust me when I say it goes beyond. Mamoru Hosoda, the director of the critically acclaimed Wolf Children, got his start with Digimon Adventure. That's how you know it's good. The human characters, unlike every other "mon" show, are nuanced. They all have backstories and personal problems they have to deal with along the way. Things like divorced parents, taking care of younger siblings, being adopted, all things kids can relate to... And the show deals with these problems in a surprisingly mature way. This is what sets it apart from the cartoony, one dimension (though still entertaining) characters and formulaic episodes in Pokemon and its ilk. And unlike a lot of kids anime, it doesn't do that whole "smurfette" bullhonky, either. Girls (like I did) will love Digimon, because once Kari enters the cast, the girl to boy ratio is pretty even, and there are multiple female characters girls can relate to that go beyond shallow stereotypes (although Mimi unfortunately is reduced to a shallow ditzy girly girl stereotype in the dub, which is not true in the Japanese version). The show is empowering to children in general, because it's not just a game, the fate of the world is in the hands of kids, using the skills THEY know. This show resonated with kids so much, that recently they made a series of new movies aimed at the fans who grew up with it, Digimon Adventure Tri, for the anniversary. This movies are available on Crunchyroll subtitled, and will soon be dubbed. These movies star the characters in their high school years, and is aimed at an older audience, but still deals with themes about growing up, but it's more things teenagers can relate to. In addition to the subs being off-putting to younger kids (edit: the first episode was dubbed), there's some mild sexual content (sauna scenes and the like) that aren't really suitable for the kid audience, it's more 11+.
Tamers, season 3, on the other hand... I would say it's for ages 10 and up. It's a much darker, subversive take on the Digital Monsters formula. A lot of people have called it a "deconstruction" of the battling monsters genre. Hosoda was not involved, but Chiaki J. Konaka, of Serial Experiments Lain instead headed the series, giving it a more moody, psychological atmosphere. Digimon Tamers was critically acclaimed, but did poorly in merchandise sales, because while adults and teens praised the more serious tone, the intended target audience of children were too confused or scared and thus didn't want to buy the toys. A lot of the themes present in this season subvert everything stated as fact in Digimon Advenure and its sequel. Unlike the bright and colorful Digimon Adventure, in Tamers, when Digimon die, that's it, they're dead. One of the main characters going through the stages of grief after her Digimon partner is killed is a major plot point. And yet, there's also a redemption arc for the very villain that killed her Digimon, and in the end, despite how bleak things are, everything still works out. Antoher thing that sets it apart is that it's not just the humans, but also the Digimon themselves that go through character development, more so than the ones in Adventure. This one definitely skews a bit older than Adventure. Chiaka J. Konaka also being a Cthulhu mythos writer, there's a lot of scary scenes. As in, enough to scare adults. There's are scenes where a girl is imprisoned by a giant, amorphous monster that feeds on her grief, and psychologically tortured, while being kept alive, and she even attempts suicide. It's also more violent than Adventure, most notably, a Digimon is impaled straight through the chest. There's no blood, just data, but it's still very gruesome. It's definitely more for ages 10 and up.
Now a brief overview of the other seasons... Season 2, Digimon Adventure 02 is kind of mediocre, though the redemption arc of one the major villains and their motivations are interesting. Season 4, Digimon Frontier, is radically different from its predecessors in that the humans transform into the Digimon, and it's a lot more lighthearted than previous seasons, and kind of mediocre overall. Digimon Data Squad I have not watched, but I heard is dark and more for older kids, similar to Tamers.. Digimon Fusion/Xros Wars I have not seen much of, but it has a lot of throwbacks aimed at people who grew up watching previous seasons.
I will also say that, as someone with Asperger's syndrome who grew up with this show and still loves it, kids on the autism spectrum will probably love Digimon. Memorizing all of the different forms, types, alignments and attacks of the Digimon is prime special interest material!
This title contains:
Positive role models
Violence & scariness