Man I loved Digimon both as a kid and as an adult.
Adventure - A great start with a lot of likeable characters, the introduction to the partner dynamic made the digimon so interesting and loveable. I thought it went on to long overall with hit and miss arcs (Miss: Etamon, Parts of the Dark masters) but it had some great moments (Angemon, MagnaAngemon, Warp Digivolution, the real world arc, ect) throughout.
02 - As a kid I enjoyed it because it was more Digimon, when I rewatched it later... It's absolutely terrible. Ken's character arc is good but everyone else is awful, the new kids have some really bad designs and so many things just don't make any sense. I remember even as a kid being extremely bothered and confused by Gatomon getting powered down despite being a Champion because of her tail ring and the armor evolutions I never understood where exactly they were on the 'power' level that the original series had developed (Mega>Ultimate>Champion>Rookie) it really just didn't work very well. Then the god awful ending was just so bad all around completely ruining various character development within the original series and 02. It has some pretty good moments (TK vs. Evil Ken is amazing, the Dark Ocean plotline was super interesting just never went anywhere) but overall its just not very good.
Tamers - Tamers is the best. It made the partner dynamic much more interesting with the human partners actually being able to do something besides cheering on the monsters and it evolved the idea into the mega forms being a merged form of human and digimon together. It also helped to have a much smaller main cast with just the Trio for a majority of the show and instead of them all being friends like the previous two seasons they start off as completely different people who really have to come together as a team. Rika is headstrong and only thinks about winning and sees all the other Digimon as enemies, Henry doesn't want to fight and just wants to be friends with Terriermon which causes conflict with his partner who does want to fight, and Takato who starts off as innocent and wide eyed as Guilmon is but quickly learns how overwhelming the responsibility he now has is and largely has to grow up rather quickly. Takato is the one who really brings the team together too.
Another thing that Tamers does extremely well throughout the first half is making Digivolution feel EARNED. In the first two seasons it was just a natural order of things, the kids get in trouble, the digimon grow stronger and beat the enemy and win the day but in Tamers it takes a lot longer for each Digimon to reach their champion form. Interestingly Terriermon can Digivolve right away but because Henry is afraid of it and he doesn't want to fight he can't control Terrriermon properly when he does evolve its actually showing evolution in a completely different way. It's not like Skull Graymon being an evil Evolution like in Adventure but it shows that a Tamers lack of resolve and experience can lead to a Digimon that they cannot completely control which is extremely dangerous. Rika on the other hand despite having a ton of experience and control over Renamon can't make her evolve because Rika doesn't care about Renamon like previous partners just kinda automatically did in the first two series. Rika even blames Renamon for the problem constantly despite not recognizing that she herself is largely the problem, it makes for a great character arc early on and certainly the most well developed character of the series. Takato despite being the main protagonist actually sees his partner evolve last because he starts off being a tamer much later then Henry or Rika and whats interesting is that when Guilmon evolves Takato is honestly quite scared, despite Growlmon being the same innocent and lovable dino/dragon thing he was, Growlmon is much much larger and rather viscous looking.
The second half of Tamers isn't as good as the first but its still really good overall. The Digimon World just doesn't feel as interesting or exciting as in any other season and really just feels like a place the kids had to go to progress the plot but they never find anything new or creative to do with it outside of the plot itself. That said this is where the plot starts to get pretty dark (although throughout this point the overall series of Tamers has been much darker and realistic compared to the first two series) with Impmon getting to finally evolve into his Mega form but at the cost of having to kill the kids who have done nothing but try and be friends with him leading to the 'big' moment where he kills Jeri's partner Leomon and unlike previous seasons... Leomon isn't coming back. This sets off the rest of the arc with the D-Reaper becoming the most interesting villain of the series (although not as well developed as I think it could have been IMO) largely by abducting Jeri who is in a depressed/suicidal state throughout most of this final arc, this also kicks of the Biomerging I mentioned above which was such a great way to give the kids Mega's and its nice that its all of the main cast for once not just 1/2 kids that get a Mega (granted theres only really 3 main characters here but still) plus Ryo and I liked how Takato's friends and Henry's sister all get partners as well and do contribute to the fight even without Biomerging themselves. Plus Impmon gets to have a redemption arc that is actually handled quite well. I really love the scene where he uses Leomons move to free Jeri but its doing that that causes her to regress and not want to be freed by him of all people since he offed her partner.
Tamers is just so good almost 100% of the time, I do not understand how anyone thinks its the worst.
Frontier - It's not horrible and it was an interesting concept but not having any partners made it feel very distant to what made Digimon so interesting in the first place. Plus it was annoying how once again only 2 of the main cast ever actually get complete evolutions with the side characters only getting 1 or 2, it also doesn't do a very good job of setting up the 'power' of each stage of evolution after the first two. Like okay it makes sense that Beast Spirits are stronger then Human and a fusion of both is stronger then that but after that they just add a bunch of others into the mix and it just gets a bit out of hand.
Savers - There is a ton to like of Savers although it starts off really really rough with some rather unlikable characters including the main character (at least at first) but after the first arc once all the introductions are made it really starts to get interesting and gets pretty great and has a lot of darker elements. It's also great that once again all the main characters have complete evolutions. Also the
second opening song is amazing. Savers is extremely hard to get into because the first arc is just not good but once your past that it really gets interesting and the characters get much more layered and the dynamic between them is really great. It's not perfect but I think its the best after Tamers to be sure.
I tried watching the Xros series but didn't like really anything about it from the characters, the story, art style, ect. I should try to see it eventually because I hear it has some pretty good moments but yeah just not something I'm interested in atm.
Overall of the first five I'd rank them...
Tamers>Savers>Adventure>Frontier>02
I think the reason that Tamers really wins this for me is that while its still very much a kids show it explores some really realistic characters and situations (like seriously if you had a Digimon in the real world WTF as a 10 year old would you do? That is a MASSIVE responsibility in itself but oh yeah it can Digivolve and GROW INTO THE SIZE OF A HOUSE!) and a genuine and honest darker story-line that made it vastly different then what the first two series did. It could've just be 03/Tri and it would have made a lot of people happy but instead they tried to do something different and explore what they really could do with this concept and I think they nailed it. It's not perfect to be sure, but its what kept me interested in the series all these years later and I find Tamers infinitely more rewatchable and interesting then any other series of the show.
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