this is my opinion
season 1 = awesome on its own and also kickstarted the series #3
season 2 = good first time round, not as good as the others, hard to rewatch #4
season 3 = the darkest of the lot, best plot, guilmon and calumon ^_^, beelzemon, it has cards #2
season 4 = changed some stuff up, my favorite, have watched it over and over, coolest evolution scenes, THEY BECOME DIGIMON! and agunimon, burningreymon, aldamon, emperorgreymon, susanoomon #1
season 5 = WTF REUSING AGUMON!? SHITTY NEW EVOLUTION!? A BOXING GABUMON!? D.N.A CHARGE!? NO GOGGLES KID!? AN ENTIRELY NEW NAME!? HARDEST TO REWATCH!? #THIS SHIT IS SO BAD FUCKING NUMEMON WERE COOLER.
season 6 = untranslated for me, haven't seen it, lots D.N.A digivolving sounds kinda gay..... oh well, final episode or 2 has tai, mat, davis, takato and takuya, FUCK YEA! #I don't know I haven't watched it in my language