It's top 5 for me.
No spoilers, but the first episode of season 5 has Don on some shyt because of some young shyt going on.
Part of that is the tone shifts as the culture does. Season 5 starts in the mid 60's so the show gets livelier as you see hipsters, counter culture, protestors, the youth movement start to slide in.but I started on the 5th season, when it aired in 2012, and went backwards. The 5th starts to get way more experimental, funnier, sexier, it's not a different show, but it is one of those that gets better each season.
No spoilers, but the first episode of season 5 has Don on some shyt because of some young shyt going on.