Firefly to Party of Five
4 connections
Firefly
Angel
Weyland-Utani is a client of Angel's Wolfram & Hart and also the company that made some of the weapons used by Firefly's Malcolm Reynolds in the Battle of Serenity
Buffy the Vampire Slayer
Angel was a spin-off of Buffy the Vampire Slayer.
The X-Files
The X-Files? Cigarette Smoking Man and Buffy the Vampire Slayer's Spike both smoked Morley cigarettes.
Party of Five
In the X-Files episode "Milagro" Mulder & Scully visit a graveyard where Party of Five's Diana and Nicholas Salinger are buried.