I don't usually get to rate 2 characters in one shot, but I'll try my best to explain why these 2 are some of my favorites.
Tails Doll feels very unique among all the other characters on the message board, yet not out of place. His Poly Shield ability is neat overall, and flat out ignoring damage for a moment is satisfying a lot of the time similarly to Sonic's Insta-Shield in S3K. His floatiness also makes him feel fresh when compared to all the characters with standard physics. His visual assets are wonderfully crafted, and I can never not smile at seeing him bounce around with his ragdoll physics.
Metal Knuckles... I love him. If you caught me before the latest update, he'd still be one of my favorites yet with a few flaws. As of now, though, those flaws have been washed away. Charge Glide gives MK a nice way to get around, and is a nice middle ground between Metal's hover and Knux's glide. Having Charge Climbing be optional alongside regular climbing makes it fun to plan on when you should use it over regular climbing in some scenarios, and it's very fun to be able to essentially switch between the two on a whim. Having a spike breaker passive is also a neat touch. Dash Mode meshes wonderfully with MK's kit, being very rewarding to get since it shortens wait time for Charge Glide and can even be kept while climbing.
I really love these two. Main game completed with 7 emeralds in tow, once for each. Play them. :)