I know what the award is supposed to be about.
The best player in the world 2021: Messi
The best season any player has had until now: Lewandowski
Who do you crown? Best player or best season?
Depends. Is there are noticeable gap between best player and 2nd best? And is there any significant difference in terms of their achievements? Which one has better narrative to win it this year?
One example is, back in 2013 even though Ribery was the most successful player that year (best player in his team winning treble) , the gap is actually quite huge between Ronaldo (55 goals 11 assist) vs Ribery (11 goals, 15 assist) in terms of individual stats, so ridicolously huge that its really hard to give it to Ribery even though he is having most successful year while also performing very well.
However, some might argue the gap between Messi and Lewandowski this year isn't really that noticeable, Messi being naturally the better performer of course, but Lewandowski is clearly more deadly in front of goals (41 goals in 29 games in BL) and has better narrative of winning it this year (break Muller's record).
Anyway, below is how I would use to compare contenders for Ballon D'or, in terms of stats, performance, success and narrative. Past winner of Ballon D'or usually either scores more points than his rivals, or being so far better than his rivals in 1 or 2 specific area.
Argument for Messi:
Stats: 38 goals 11 assist in 47 games
Performance: avg rating 8.52, 24 motm
(Messi wins)
Success: winning domestic cup
Narrative: carry his team to 3rd place finish in league
Argument against:
Failed to lead his team to progress further in CL, failed to win a major trophy
Argument for Lewandowski:
Stats: 46 goals 7 assist in 35 games
(Lewandowski wins)
Performance: avg rating 80.7, 13 motm
Success: winning league title
(Lewandowski wins)
Narrative: break Muller's record
(Lewandowski wins)
Argument against:
hard to find one, maybe injuries prevent him to help his team progress further in CL
I'd say Lewandowski is currently leading it. But lets say if Messi lead Argentina to win Copa, Messi would surely have bigger chance of winning.