Currently the highest life expectancy country is Japan, at 84.6 years https://en.m.wikipedia.org/wiki/List_of_countries_by_life_expectancy
Related questions
UN, World Population Prospects (2024) update for 2023: Monaco back above its prepandemic level (86y): https://ourworldindata.org/explorers/population-and-demography?facet=none&country=~OWID_WRL&hideControls=true&Metric=Life+expectancy&Sex=Both+sexes&Age+group=At+birth&Projection+Scenario=None
However, Monaco's LE has only grown by 2.5 years over the past 16 years. South Korea, at 84, increased its LE by 4 years over the same period. So, if we assume the same trends over the next 16 years, Monaco and South Korea might reach 89 by 2040.
Actually the UN forecasts Monaco #1 in 2040 at 88y: https://ourworldindata.org/grapher/future-life-expectancy-projections?tab=chart&country=USA~ARE~FRA~CHN~KOR~MCO