My favorite is Mott, mostly because of personal investment. There are other Lions whose designs I like better, but I've had a particular history with Mott that boosts him ahead of the others. Basically, years back, I had Mott living in one of my New Leaf towns but decided one day to move him out and replace him with some other villager whose design would better fit the town's theme. As such, when he asked to leave, I gave him permission. As the days before his departure passed by, I sort of regretted my decision, but he never again asked me if it was the right thing to do, so he did end up leaving. I was kind of bummed, so I decided to eventually reclaim him. Months later, maybe even a year later, I got another copy of the game to make a separate town with a new theme, this time a town themed on a circus, and of course I'd need a lion to be one of the stars. So, I reclaimed him and he's been a resident of that town ever since.
My close second favorite is Lionel because of his military design in New Leaf and noble gentleman appearance in New Horizons being distinct and interesting, as well as his Smug personality. I'm also fond of Bud and Leopold. Elvis is pretty neat too; his design is pretty stock standard lion which has its appeal, his scars add a nice touch, and his overall royal theme in terms of his clothing and interior design of his home is nice.
I will say that I also always liked Aziz's design from the GameCube game, the blue mane really made him stand out. That said, I never had him in my town.
Lion is an overall pretty good villager species as far as I'm concerned.