Hear me out: Lakers vs. Celtics, 1987 NBA Finals (especially Game 4) and events after.
Celtics were a little older, a lot more beaten up (Bird had some ankle/back problems, McHale broken foot, Walton's injury problems returned), and not as deep (K.C. Jones running like an eight-man playoffs rotation in the regular season

). Bird especially made sure they still had what it took to get back to the Finals though. Magic was already great, but he raised his game to another level that year and didn't really have a bad game all playoffs. Lakers had really good younger players (Byron Scott and James Worthy were 25, A.C. Green was 23) and were more or less healthier and deeper than Boston. Y'all know how it ends, but damn if Game 4 wasn't one of the most crushing single game Finals Ls in NBA history and a microcosm for that series as a whole. Just not enough.
Lakers win in six. McHale is never quite the same player. Bird's back gets really fukked up. Danny Ainge gets traded. DJ ages out. Len Bias dies. They're not good/deep enough to hang with Detroit or Chicago anymore. Reggie Lewis dies in '93. The Lakers stay at or near the top until Magic's retirement, which is a few years after Boston's relevance as a contender. Celtics didn't recover for a long time.