Quoted By:
-GREAT-
1) Adventure
Great character development, the crests being the key to their growth as people really cements this as the best digimon season.
2) Tamers
Pretty good plot and characters, Guilmon is fucking kawaii, best final evolution theme [ONE VISION]
-GOOD-
3) Savers
A slightly more mature shot at the digimon series, mainly great for watching the main character punch digimon in the face
4) Xros Wars
A main character that isn't a hotheaded shounen hero for once, actually being more like a tactician. That digimon power levels don't really exist in this takes some getting used to.
-PRETTY OKAY-
5) Frontier
Not having partner digimon makes this not feel like digimon at all. Otherwise though the character development in the first half of the series is pretty great, but expect your interest in the series to completely die out in the second half due to how the evolutions in this series end up.
6) Xros Wars 2 - Hunters
A /really/ villain of the day series, definitely would still recommend watching if you've already watched Xros Wars for the callbacks and how awesome Shoutmon looks now. Really though both Xros Wars felt like they had such a good gimmick but failed to do anything interesting with it (Xros-ing digimon into anything you can imagine)
-BAD-
7) Adventure 02
Shit on everything Adventure set up, only worth watching just to "see how it ends", except it's going to end terribly. At least Takeru is badass in this series.