Alexander Wurz from 1998 is a good case for me. His first full season, a string of good points and was a legitimate match to Fisichella that year. Whatever happened after that, is a mystery. He was nowhere near Fisichella and dropped out of F1 after 2000. Which is not what you would have expected based on 1998!

What about Ralf Schumacher in 1999. First season for Williams, utterly destroyed Zanardi and made people talk about him like future great. Like Frentzen, Ralf Schumacher also made his name that year. Ralf still had good seasons in 2000, 2001, but not quite as standouts and after that his career started to decline comparatively.

For whatever reason Barrichello stands out from 1999 too... But for different reasons. It was not his only impressive season and he had good seasons before as well. But it was the last season in which he looked like a future star. And he finally had a good car. Got podiums, got a pole, led races. Unique season for Rubens. Other than that he usually had just merely unremarkable midfield cars (before 1999) or he got consistently beaten by team-mates (from 2000 onwards).

Felipe Massa, 2008-2009 (so that contains two seasons). Before that he was either unremarkable in midfield (Sauber) and got beaten by team-mates even if he looked promising. But in 2008 he suddenly was a title contender, beat Raikkonen, and almost won WDC. His 2009 was decent too until accident, though not so spectacular in qualis. Since then has been getting beaten by his team-mates. But 2008 stands out massively.

Overall... I think in this thread any driver's best season can be added. Because all the other seasons would look inferior, so that best is the benchmark. Just maybe the performance differentiator is not big compared to other years and there are always factors, which influence the outcome. What about Alboreto from 1985? Regazzoni from 1974, almost WDC and beating Lauda? Hunt in 1976, only season he was ever a realistic title contender? Hulme in 1967? Pironi in 1982 - almost WDC after getting a trashing by G.Villeneuve in 1981?