Differently Morphous is my favorite of his books. It is incredibly funny and the humor somehow doesn't take anything away from the moments that are meant to be more thrilling or serious. I thought it handled the underlying theme of Political Correctness very well, probably because it mostly stuck to being descriptive rather than prescriptive. It was my first audio-book(turns out Audible got what they wanted out of the exclusive) and Yahtzee does a good read, though one of the book's funniest chapters is somewhat hampered by two characters having an extended back and forth with the same voice. I'd strongly recommend it to anyone.
Mogworld is a close second. The premise is well fleshed out and the book is packed with so many great moments where your expectations of the setting are subverted to great effect. One of my favorite examples of this occurs right at the beginning when a great Necromancer's grand victory of breaking the very laws of reality to raise an army of the undead to do his bidding is somewhat deflated when the undead expect to be paid for their service, turning his much rehearsed victory speech into an impromptu negotiation. The book has a bunch of great characters that were incredibly fun to read about and can be surprisingly complex given the comical nature of the book. The book was a very funny and very enjoyable read.
Will Save the Galaxy for Food was good, though I think it went a bit overboard with foreshadowing its story twists to the point that I began to think the twist was going to be that the twist wasn't going to happen given how heavily he kept hinting at it. I thought the characters were weaker than those in Mogworld, probably because in Mogworld the characters start as cliches and stereotypes that become more human as they further developed and characterized, were as in WSTGFF the characters start off as human and mysterious then become more cliched and stereotypical to fit their roles in the plot as it unfolds. The book is still a lot of fun and it has convinced me that every scifi RPG needs to have a bureaucrat class.
Jam was the only book that I didn't really like. A lot of that is just down to personal preference rather than anything objectively wrong with the book. The bits with the mall Hipsters drag on for far too long and I think Yahtzee greatly overestimated the staying power of jokes about how they misuse the term "ironically". My other criticism was that deaths happen so frequently that they quickly lose all impact, which shouldn't be the case when people die in a story. The book just left me feeling numb and unpleasant.