On the whole - Xanathar's.
As others have said, the downtime rules and the massive and extremely useful expansion of the PHB spell list are what makes Xanathars' stand out. The subclasses here are not always incredibly interesting or powerful, but they do often fill obvious niches in the game world that the PHB left begging for want of space. Divine and Shadow sorcerer, Celestial warlock, Grave cleric, Ancestral Guardian barbarian, swashbuckler rogue etc etc. Unfortunately some of them are distinctly mediocre in terms of mechanics (hi there Mastermind!) which is especially unforgivable given some of them were effectively reprints from SCAG. There should have been some rebalancing done in the interim.
Big pluses for Tasha's are the alternate class features, and starting to address some actual game balance problems between classes in a stealthy kinda way by, for instance, giving Sorcerers effective additional spells known via the subclasses, and giving Rangers some alternate features (that are much more useful than the original ones). The puzzles bit was an utter waste of a LOT of pages on what seems like a really niche playstyle - and Tasha was much less funny than Xanathar. It's a bit more experimental book than Xanathar's, and i think it shows. The highs are really excellent (my next PC is an Order of Stars druid...) but compared to Xanathar's there's a lot higher percentage of lows where the experiment fell flat on its face.