I feel like ribs don't get enough appreciation as a tickle spot... it's one of my favorite places to tickle.
When you're tickling someone, there is sometimes -- not always, but sometimes -- a magical moment when their hitching breaths finally culminate into a laugh. It's like when you're starting a fire using nothing but sticks, rubbing the wood between your palms until your arms ache... and then you see that very first spark. A promise of roaring fire, as long as you tend it carefully!
When you're tickling someone's ribs, every gasp and shuddering breath is right there beneath your fingertips. Before their laughter is even audible, you can see the smoke starting to curl, feel their breath jolt and huff as you probe for the best spot to focus your attention.
You see everything, you feel everything; the blush on their cheeks, the movement of their lungs, the smile in their eyes before it has even touched their lips. You both know that once that first spark of a giggle or even a squeak escapes, it's all over from there -- and when the spot is their ribs, you get to watch that beautiful moment as it happens:
Their mouth is still closed, but you hear a giggle hum through pursed lips. There, right there. Your fingers work into that spot, tickling away as their grin breaks into an open-mouthed fire of laughter, and what an enviable position to be bathed in the warmth of such joy! You coax that fire into a raucous blaze, playing up and down their ribs as you watch and feel their enthrallment, the heat of their body, their movement and sweat, laughter and life; it engulfs you as much as it does them. It's beautiful, that kind of shared sensation. It's magic.
[no minors, don't tag s/f/w please]