I never actually got the hate for Frontiers. Yes, it was a radically different approach, but the general concept of human and Digimon relationship bonding was still there, just a bit more literal in this case. To me, Frontiers, though not the pinnacle Digimon season, was a good, decent romp through the series. It had interesting villains and a unique plot to the "save the world in time" (as the Digiworld literally fell apart). I didn't care too much for the twins, but then again, I had just about lost patience for the angsty, pretty boy at that stage in time.
My least favorite is and will always be Digimon's second season. Oh, it started off great. The first 26 episodes were fantastic and I adored Ken's character development (back when I DID enjoy the angst, ridden pretty boys). To see the older Digidestined as guidance was a thrill, especially Tai who really proved his maturity. Then sometime after Ken joined the main cast, the plot fell apart. There were simply too many characters and little time to show them all, resulting in a convulted mess that ultimately resulted in a cheap ending where the villain was defeated through children's wishes. And a finale that was random to several of the main characters.
Digimon Tamers is my grand favorite. Yes, it did ignore the secondary Digi-kiRAB in favor of the main trio and Ryo is a big Gary Stu (Yes, I'm aware of his multiple canon jumps), but it held the most beautiful animations (I adore how they drew the kiRAB' eyes), and the deepest, darkest plot. It was compelling, it held visually pleasing character and Digimon designs, it molded the world of Digimon and the human world well. It was just great, period.