It's legitimately difficult to decide. After season 4, they were all horribly written, punctuated by truly offensively bad moments.
Season 5 had Sansa handed off to Ramsay like so much meat (in a political play by Littlefinger that made less than zero sense), robbed of any agency and empowerment they'd bait and switched us with by her Season 4 Maleficent exit, instead having her fill the role of Jeyne Poole, a character with one of Martin's crueler and grosser fates.
It also had Barristan's laughable death, for which the actor wrote a harsh letter to the showrunners because they were such disrespectfully poor adapters.
I've done my best to block out the minutiae of the TV series' latter half, tbh, so I'm just going to stop here and vote for S5 rather than suffer remembrance.