Frontier is pretty decent. Not the best season, but definitely still watchable.
Indeed, I felt it got a bit too much hate. I went in expecting something quite bad and it turned out to be decent. (watched the dub, not seen the JP ver - the dub op is awesome imo)
As for the ones after frontier
Savers/Data Squad is good. Though This is the one season where I openly prefer the JP version but I don't mind the dub either. Just the op for the dub sucks and looks rushed as anything. Also episode 15 (which is one of my favourite episodes) is better in the JP version due to music/atmosphere/etc (though the dub did it better than I expected it too considering how it was. Anyone who's seen this episode should know what I mean I hope lol)
And as for the Xros Wars/Fusion. I haven't seen the dub and I've only watched the first 30ish episodes roughly of Xros Wars in Japanese. It felt more samey than your typical monster of the week anime and at times it felt more uninteresting than them as well. I'd say this is the first season of Digimon where I actively got bored at it for more than 2 episodes.
Some of the later stuff looks okay from what I've seen (via YT)/read but the start is just so bleh.
I also didn't like the whole Xros Wars thing, mainly because it's like one Digimon with different variations on him (Shoutmon) for most of the episodes I watched. That might have been one reason why I got uninterested in it. The whole 30+ episodes I saw focused on Shoutmon and the kid (forget his name) almost exclusively. And I didn't feel it would get any better when I stopped watching.
(though truth be told, I stopped watching because I completely forgot about the episodes coming on each week. That's how not interested in it I got.)