My favorite free F/M story, not written by myself, is "Tickle Toy of Mary-Kate and Ashley (F/m) by Milagros317". It's an oldie, but to this day it remains one of my all-time favorites.
Any stories from <a href="https://www.ticklingforum.com/member.php?30303-parpignollord">parpignollord</a> and <a href="http://www.ticklingforum.com/member.php?91922-flyers21">flyers21</a> are my favourites.
The posters above have all sited some really fantastic f/m stories by some really good writers. "All time favorite" is a tall order — I feel like I'm going to "diss" someone by not naming them. PeterVincent's "Jenny the Clown" stories have some great f/m tickling; Cosmo AC's "Stocksville" series has some great f/m tickling; TQLR's 2-parter "Tommy's Summer" (his only f/m story) is really good. I also like Jaynin's stories — it's a fantasy setting (don't know if that's your thing) but some of the stories are f/m focussed… I could go on. But… an older story that really struck me was called Edmund and the Night Visitors. It was on the old "Josie the Ticklish Pussy Cat" site, and was written by someone called "Dark Writer." don't know if Dark Writer is on the TMF (or on here under a different name), but I recall he or she had a couple other great f/m stories called "Island of Delights" (or maybe "Isle of Delights") and "The Surprise."
But "Edmund and the Night Visitors" is well worth your time. Jerry67911 and Wildman reposted it several years ago. It's right here.