Seska hardly betrayed the Federation. She was a Cardassian operative who was undercover to infiltrate the Maquis, before being forced to serve on a Starfleet ship by circumstance.
Ro was the bigger Federation traitor, though it definately was far more personal for Picard and the gang.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)