Easily AJ Styles for me. It's not really a far competition since he got two quality reigns, and had the added benefit of taking the Championship of Jinder Mahal, which is always a plus. I kinda stopped watching WWE altogether around Extreme Rules because I was more than a little bitter with how they were treating Asuka, so I kinda missed out on the Styles/Joe feud and whatever Styles did after that til he dropped it to Bryan, but ultimately I liked the feud with Nakamura quite a bit, even if their Mania match was awful. After that probably Daniel Bryan, just because Kofi's is too early on to properly judge. I've really enjoyed Bryan's character work since winning the Championship, and the added touch of the Hemp Championship was a fantastic touch that certainly adds to his reign. But his matches didn't do much for me in this period, I thought the Lesnar match was tonal mess and kind of a structural retread of the Styles match the year prior (which would be fine, if they match wasn't set-up exactly the same as it was the year prior too), and the match with Styles at the Rumble was mostly a slog with an unsatisfying finish. Liked the match with Kofi at Mania, and haven't seen the rest of his defenses/big matches, so it's possible those would change my opinion. After him, Kofi, just because of the context surrounding it, and the amount of build to his moment. Then Ambrose, who had some quality promo and matches but was too short to compete with the others.
The other reigns were trash. Cena's my favorite of them, if only because the Rumble match with Styles was great. Wyatt should've never won the Championship, but at least he had the common decency to lose it immediately, Jinder Mahal held the title for way too long without ever really improving, and almost all his title defenses ended in the same exact manner. Oh and Orton held it at one point, I guess I'd put him below Cena but above Wyatt since I actually like Orton as opposed to Wyatt, who I've pretty much always disliked until a couple days ago when he be a children's TV host.