My favourite of the series is Mossflower, for a few reasons.
The Greeneyes family had a really interesting dynamic to me, for a start. Though we only saw him in action briefly, Verdauga to me was another one of those minor characters that managed to capture my imagination far more than I think Jacques may have intended. We know he conquered Mossflower and put down a rebellion led by a badger, feats I'm not sure were ever repeated in the series. However, in his time ruling it seemed to me he was portrayed as a reasonable, if strict source of authority, and was making a genuine effort to prepare his son for a future of leadership. He also had a tantalisingly vague backstory as being the son of one King Mortspear who ruled in the Northlands, which suggests the Greeneyes family has a long legacy of conquest and rule.
Tsarmina was a great main villain of the piece. Murdering her father and imprisoning her brother for the sake of seizing power really set up quickly just what sort of ruler she would be. I also loved how much of a warrior she seemed to be herself, since a lot of villains tend to be shown up as cowards at heart. Not Tsarmina, she's leaping over walls and chucking spears at squirrels and genuinely living up to her warlike family's legacy. Gingivere's also an interesting kettle of fish since he clearly never wanted power in the first place, but might've actually been able to end the oppression of Mossflower had he been able to take it before his sister.
I enjoyed these characters and their interactions quite a lot. It's safe to say the wildcat characters I've written on this site owe a lot to the Greeneyes. I think Jacques may have tried to replicate that kind of dynamic again in the Felis family, from High Rhulain. A warlord father and two rival siblings, with the addition of their mother. I don't think this was as successful though, since the characterisation to me wasn't quite as nuanced.
Anyway, my gushing about the Greeneyes aside, I also liked that this story had the usual power dynamic reversed, with the villain characters having the powerful fortress and the heroes having to make do with hiding in the forests and in a secret lair. It gave it an odd sort of 'Robin Hood' quality to me, especially with Gonff's characterisation and the bumbling nature of minor villain characters like Cludd, who could be an excellent stand in for the Sheriff of Nottingham. It also gave us Mask, an unusually quiet sort of hero that was all about deception instead of strength. His outfoxing of Fortunata I thought was pretty well handled seeing as she was supposed to be quite cunning herself. I have to say though, the method of her execution was quite dark, since she was lured out into the woods only to face a firing squad.
Overall though, I think things like that made the story a lot stronger and more memorable. Sad moments and character deaths, and the overall atmosphere of danger and oppression made the triumphant moments and the hero's victory seem that much more well-earned. It also had a variety of interesting minor characters and subplots, like Ashleg's eventual desertion, Argulor's strange obsession with trying to eat him, the comic duo of vermin chasing Martin, and the introduction of the Loamhedge Abbey characters, which tied the series' canon together nicely. It made the story's world feel more alive and full of interesting goings-on outside the main plot.