Having only played Sword (I originally wanted Shield first because someone IRL was planning on getting Sword but there weren't any left), I can say that I love the leaders we got. Gordie was weirdly comforting during his gym challenge, and fighting Bea was just a lot of fun.
My favourite gym challenge was probably a tie between Gordie's, Milo's and Nessa's. Milo's was by far the easiest, so it was a fun introduction to the concept of challenges, and I love puzzle's, so Nessa's was a blast. Similarly, although avoiding the pits was really tough because the detector was unreliable, I loved navigating Gordie's challenge.
My favourite gym leader in general is Raihan, without a doubt, though I adore them all. Kabu, Opal, Piers, Bea, Nessa, Milo and- yeah, no, they all pretty much tie for second place. This region has the best gym leaders in terms of variety and personality, imo.