Depends what you're into
Tickle Abuse
Beg Harder is a classic. I'm primarily into M/F, not F/F, but this is incredible. Any clip with Diane's upperbody is a classic.
Licklish Indian pits - Prya. Amazing
No Rack for Tracy - Outstanding upper body tickling. Cannot recommend it enough.
Frenchtickling
Helia is an amazing clip. Any upperbody clip with Camille is cool too. These are old clips, so the res might not be the highest.
Kelly's upperbody tickling - Fantastic lee
Flavia's upperbody too is a good one, though I'd recommend downloading the preview before buying the clip - her laugh isn't for everyone (I liked it though)
Rook's Media did some fantastic upperbody clips with Lo Lo. Always though she reacted better when tickled by a guy. Again - lo res though
Zen Tickling - Anything with Foxy Roxy is a must-buy clip. His clips with Amber were great too.
Absolutely Tickled's series with Maria gets a rec too. Go through the preview clips and see the setup which you like the most.
Tickle4torture.com - Elan has the most ticklish armpits ever. In general, Elan Kane upperbody clips are gold. There's another one by Tickle Chicks - Elan Kane is pinned and tickled. It's awesome.