I gave up partway through Frontier. I was obsessive during the first three seasons, 4 just killed it for me because Dejimans will always be about partners, not becoming the Digimon.
>Adventure
First season I watched back in 5th grade obviously, so it got me into the series, but after 02 and Tamers, I didn't care much for it outside of Hikari's story and Gatomon+Wizardmon.
>Zerotwo
Best monster designs out of the three imo, but the story was all over the place, and the kid's outfits were atrocious (kind of a huge contrast compared to the Digimon themselves). The worst part was the Evolutions. Armour evolutions played a huge part in the beginning, then just stop mattering out of nowhere when the Digimon get champion and ultimate forms. It also made no sense to have a new leader who happened to be a complete dumbass, when Takeru already had lots of experience with the Digital World.
>Tamers
Still liked Zerotwo's monster designs better, but this had the best story and characters out of all three.