Season 1 was a good “Year One” arc. the most notable moment being Ronnie Raymond Firestorm.
Season 2 is the next speedster villian. I am glad that Zoom had a black suit as opposed to the traditional reverse-flash style. also the Harrison Wells of this season would inevitably start what will become the Council of Wells.
Season 3 is the next speedster villian after that. While Flashpoint did reverberate across the flarrow-verse, it seemed like it was jammed in just to do it. i like that there were consequences even after “fixing it”, but this is also the first appearence of a time hack: they know Savitar kills Iris, so this season’s Wells uses his camoflauge tech to sub in for Iris.
Season 4 can be summed up best by what i like about the Thinker: that he is not Godspeed (or any other speedster for that matter). Elongated Man has been a true gem, so double kudos for introducing him. He was also the first weak point in the Thinker’s plan since he needed his body to handle the damage his thinking does. but as the season progressed, I kept wondering how can you beat someone who has thought of everything. And the answer was always the same: you must betray him. And as we saw, recruiting his wife was the final step in defeating The Thinker.
Season 5 ultimately brings us back to Thawne since the whole season is about him trying to get out of being executed. of which, we really don’t know what he was convicted of, or how he got caught. but he proved his thesis of the timeline is malleable at the cost of this version of Nora “XS” Allen. The Flash disappearing was a moment referenced in season 1 that if it remained intact, the timeline is preserved. Which allegedly is related to Crisis on Infinite Earths. 'Nuff said.
Ultimately, my vote is season 4 because it kept me guessing.