tl;drGiven that he is semi-retired now, here is my ranking of all his seasons as a professional footballer:
Horrible from start to finish, only saving grace is the Finalissima at the end of the season
First real season and first WC
Completely disengaged for large parts of the season, a big drop off from his last season, he failed to show fighting spirit in La Liga or the CL, resulting in the most humiliating season in the Barca shirt. Still made 20+ goals and assists
If he had somewhat kept his form from the first half of the season, along with the glorious WC, this could be a little bit higher
Starting to become the club's focal point
Already among the best in the world
Horrible first half, then produced an almost vintage run of games for 3-4 months but then failed along with the team on the finishing line. Extra points for the Copa though.
The odd season during his decade of dominance, injuries, the upcoming WC (which he kinda underperformed) and changes on the team produced a meh season. Still pretty good though for other players' standards.
Again another odd season, coming fresh from one of his best seasons ever, playing along prime Neymar and Suarez, it felt like he was not that motivated at times. Still he scored and assisted an amount of goals which could be considered career defining for other superstars, however the CL run left much to be desired
Probably the best season he could have had playing under Valverde and after the massive downgrade of Neymar -> Paulinho. He showed signs of his clutch gene of his past against Chelsea, but then he and the team collapsed against Roma. Plus, the 2018 WC was also a big disappointment, so this season could not have been placed any higher.
First season as a 10, treble, goals in crucial matches, exceptional dribbling etc. His first truly career-defining season, but still not a complete player
One of my favourite seasons of his. If he and the team did not collapse against PSG and Juventus, this would be higher. Despite not winning the league, this was one of his best La Liga seasons. Not only did he score a shit ton of goals, but the quality of most of them was astounding. He mastered the free-kicks this season, while also retaining some of his elite dribbling ability.
A big step up from his previous season, already being put in the GOAT conversation. Insane goals, insane dribbling and many clutch moments.
This season has been analyzed to death. The most complete version of Leo, producing vintage performances night after night at 31, while being the best FK-taker and one of the best creators on the planet. All of this, while playing in one of the most mediocre teams of his prime. What could have been...
The season that he truly mastered the art of goalscoring, scoring a brace for like 25 games in a row lol. In the meanwhile, he played in a dysfunctional team, which prevents this season of being any higher.
Here Messi is at the peak of his powers. A goal-scoring machine, one of his best seasons as a creator and still a fantastic dribbler. All this while winning the treble. The only reason keeping this season from getting higher is that he did not have that many iconic moments in the CL.
73 goals. 32 assists. Did not win any meaningful silverware, did not manage to beat Real/Chelsea, but in my eyes was the most unstoppable force football has ever seen.
When considering everything, probably the best season in history. Best scorer in the world. Best dribbler in the world. Best creator in the world. Most clutch player in the world.