For me it was 86-87. I went to my first game in in the 75-76 season, but didn't start going regularly till 78-79 so from then I'd had to watch us being second best to celtic, aberdeen, and even dundee united. Then in 1986 Souness arrived and everything changed overnight, resulting in us winning the league that season for the first time i was old enough to enjoy it. A close second would be the year we beat Leeds United and had a good run in the Champions League.