Adventure and Tamers hold up excellently. Maybe even better now because you can have the internet and watch the Adventure stuff in the proper order, without the butchered movie, and you can watch the whole series without the awful dub. The series was so good it was good even with that dub, so that's saying something.
Also, if you're autistic enough to be into the games, then you can get right into the deep lore of the franchise. For example, at first glance, Digimon World seems entirely unrelated to Adventure. But it's not. It was released after Adventure in the west, but well before Adventure in Japan, and takes place well before. Then there is a manga, V-Tamer 01, that takes place in an alternate universe (like how most of the anime do) but has crossovers, and with the multiverse stuff, stuff that happens in one universe does affect others. Stuff that happens in V-Tamer 01 was referenced in Adventure the whole time, but of course kids, especially outside Japan, wouldn't get that.
There are also the WonderSwan games, which directly tie into and connect the Adventure universe and the Tamers universe, and are where all the real deep story is. But only the first two of the games are translated. One only recently. The first game Anode/Cathode Tamer (had two versions like Pokemon, then a third enhanced version, like Pokemon) has pretty simple catching and training mechanics and your digimon can't even evolve, but the battles are grid based and I like that. The second, Tag Tamers, has better raising mechanics and you can evolve and merge your Digimon (in fact, you'll need to do that a lot), but the battles are simple three on three non-grid based stuff, so I find it less fun than the first game. I recommend both to fans though. Also, it turns out that Tag Tamers was actually very heavily referenced in Adventure 02, and nobody outside Japan could have known since the game was never released anywhere else. I'd almost go as far as to say that Adventure 02 is as much a sequel to Tag Tamers as it is to Adventure.
There are three more WonderSwan games, D-1 Tamers, Brave Tamer, and Digimon Medley. D-1 Tamers and Brave Tamer apparently get into much deeper lore about the origins of the multiverse and directly connect the Adventure and Tamers universes. Digimon Medley apparently has you do stuff again with characters from all three seasons, but nobody talks about that one, presumably because it has a less serious story or something. Also the Digimon Battle Spirit games are fighting game spinoffs of this series, which is why they have the same villain. Of course those were released on GBA, but the first one had an enhanced edition, Battle Spirit 1.5, which is still WonderSwan exclusive. Battle Spirit kind of sucks though so I don't care much. But I do hope we don't have to wait another 20 years each for translations of D-1 Tamers and Brave Tamer.
The Japanese movies that were frankensteined together into the American disaster were actually good. They easily could have made two American movies instead, and I don't know why they fucked it up. The first one, just called Digimon Adventure, was a short film that came out before the anime and is heavily referenced in the anime, so treat it like the first episode, basically. A prologue. The second, Our War Game, takes place after Adventure (and after the first WonderSwan game, Anode/Cathode Tamer), and is just generally a good movie, with a decent story and cool animation. They cut so much from the latter movie in particular, they could have just put these first two together without cutting anything and sold it as a full length animated movie in America. Instead they cut so much from them and then added a third butchered movie as well, even though it had a whole different cast of characters.
The third movie they added, The Golden Digimentals, is the most bizarre case, as it's the closest to being a regular feature length film, but it's about the cast of Adventure 02, while the first two movies obviously are not. But it has a substantial subplot about the cast of Adventure 1, except they cut that subplot out of the American version even though it's the only part of the movie that's about the same characters as the other two movies. Anyway, it's actually pretty good. I don't know if it's as good as Our War Game, but the Japanese version at least comes close. If they thought it was too short, there actually was a second Adventure 02 movie, Revenge of Diaboromon, that was about the same villain from Our War Game coming back, so they could have added that on, since it's kind of short, and packaged that with The Golden Digimentals to make a whole Adventure 02 movie for America. They could have had two good movies instead of one awful one. No idea why they did what they did.
Tamers is pretty standalone (until you pay close attention and see references to Adventure 02 that are actually references to the WonderSwan games that connect the two anime universes), but most seem to agree it's the best, if not tied with Adventure. More serious and darker in tone, with even better characters (though I'd argue the characters are what make Adventure good as well). Also there are like two movies but they're nothing that will leave you confused if you miss them. They're pretty good though, and if you do watch them, they have some good autismbait deep lore stuff.
The fourth anime was Frontier and people don't care about it. Much lighter in tone than Tamers, and even Adventure. Story is a bit simpler, characters are simpler. Also the kids turn into digimon instead of having partners, and that's weird. I didn't like it as a kid because it felt too different, but rewatching it more recently, I'd say it's actually pretty decent, but not as good as the previous ones. A lot of people like it better than 02, but I don't.
I skipped talking about 02 because I like it but most people don't. I say just watch it. I like the world and characters and think the plots they get into are actually pretty cool. Some plotlines don't get the attention they deserve, but the ones that do are still good.
There's a movie called Digital Monster X-Evolution. It has PS2 cutscene level CG animation. That's because it's actually a sequel to Digimon World 4, which nobody seems to like. But still, it's there. The movie is actually okay, and does introduce some important stuff to the overall lore. I'd give it a watch. You can understand it without Digimon World 4, though I hear if you do play it the movie is better because you know more about the characters and stuff.
Digimon Savers has a different artstyle that again turned me off of it like Frontier's kids turning into Digimon did. But it's actually good once the plot gets going. The main character is older than the other series, but it's not like it's as "mature" as Tamers, or even Adventure. Much more than Frontier though. Most people would probably rank this third, behind Adventure and Tamers.
Xros Wars is shit but it lasted for three seasons. Each season is better than the one before it, at least, and I'd say the final one is actually decent. It has a new main character, but the main guy from the first two is still around and important. At the end it Gets into multiverse shit, which I like, just because I get to see characters I give a damn about show up and do stuff.
The last couple of episodes are probably the only real reason I have a positive feeling of it. Seasons 1 and 2 were lame though.
I only just barely started watching Appmon. It seems okay so far.