2006 - I don't know if he had the best rookie season of all-time, but he's in contention for it. Iconic season to kickstart his career
2008 - Peak season, his best showing. Swept awards
2009 & 2010 - More peak seasons. They're a bit more "boring" than the others, but to me Ovechkin is who he is in large part because of his 3 year peak, so it's hard to ignore any of the 3.
2025 - Beating the all-time goal-record is certainly iconic. Hard to ignore it
Runner-ups:
2018 - I really wanted to find room for the cup/smythe win. But honestly - it was a bit of a "run of the mill" cup/smythe win. Great performance and all - but nothing that stands out "all time" - a lot of players have won cups and smythes. The rookie season, the all-time great peak, the goal-record - those are more iconic to me.
2013 - This was important because people thought he was "done" after 2 slower years...and even halfway through this year, he was sucking, and Caps were out of playoffs...then he goes onto close to a ~goal per game pace for last ~30 games or so, helps Caps make playoffs, win division and wins the hart at the same time. Heck of a turnaround for him. I'd have this one seventh.
Amazing career.