Sonata strikes me a lot like Pinkie. Fun loving and a bit of a goof. The ticking to her is part exciting and part sensual. Using softer tools on her blue soles will get her really squeaking out.
Luna for me is the prime pon pon when it comes to tickling being as much sensual and arousing. She WANTS the harshest tickle tools. She wants the pinwheels and brushes and combs. A near immortal moon princess has felt many sensations in her life. Someone who will actively let her feel them in a pleasant and intense way is rare and valued to her.
For Pinkie it’s 90% fun! The element of laughter loves a good tickle. Both in dishing out and getting it. You can be really tickle-harsh to those soles. She’s absolutely just keep wiggling her feet in a loving desire for getting more and more of it.
And then there’s Sweetie Belle. For young Belle it’s 99% excitement and fun. Sweetie is outgoing and adventurous. Coupled in my head canon with a budding love of sensation play and feet and indeed she’ll giggle and squeak for you. Using the relatively ‘softer’ tools is advised for good contrast.
And there we have it. My little love letter to tickle-loving ponies. ^^