The Milwaukee Bucks have been NBA title contenders, on at least some level, for the past seven seasons, dating to their 60-win season in 2018-19. This qualifies as an outsized window in the modern era, where repeater taxes and aprons have been installed to make this sort of run a real challenge.
But most importantly, they won a championship in 2021 and validated everything and everyone. Having said that, they have lost steam. They've exited in the first round of the playoffs in three consecutive years and haven't made it past the second round since the title run.