This car is owned by New Jersey State Assemblyman Ronald Zellman (recurring guest star Peter Riegert). It is only glimpsed (the camera zooms in on the personalised license plate) in episode 4.7 "Watching Too Much Televison". In one earlier episode, Zellman arrives outside a cafe to see Tony, the car he gets out of is also a LeSabre (possibly the same one).
From 5.08. Belongs to Carmela's parents Hugh and Mary DeAngelis (Tom Aldredge and Suzanne Shepherd).
A LeSabre (different colour this time) also appears in their garage in the background in episode 6.06.
From episde 6.01, used by Eugene Pontecorvo (Robert Funaro).
From episode 6.12. After losing his Maserati, Christopher Moltisanti uses this Buick LeSabre as his main transport, though it only makes brief appearances in a few scenes.
From episode 6.15, when Phil Leotardo's men take out Faustino "Doc" Santoro.
From episode 6.21, the final scene where Meadow tries to park her Lexus in the space behind this Buick.