I put all the level curves of each Generation onto a line chart.

Notes:
- I used the third version for each Generation, except Gen 1 (used RGB instead), Gen 5 (used both BW and B2W2), and Gen 8 (used SwSh).
- The X axis refers to the game progress, which I split into the start, when you get your starter, all 8 Gyms (Trials in Gen 7), and the Elite Four (Champion Cup in Gen 8).
- SwSh technically has 6 "Elite Four" members, but I was able to condense it down to 4 because some of them share the same exact levels.
- "Ace Level" refers to the highest level Pokémon that the Gym Leader/Elite Four member/Champion uses.

Observations:
- XY, surprsingly, has the highest E4 and Champion levels out of all the games, with RGB in 2nd and SwSh in 3rd.
- Crystal has the lowest E4 and Champion levels, with BW in 2nd and Emerald in 3rd.
- The point where each game has the most similar levels (apart from the start) is Gym 5, hovering around 30-37, with only RGB being an outlier at 43.
- The point where they are most different is either Gym 8 or E4 1, depending on the way you look at it.
- The biggest jump between the last E4 to Champ is in SwSh, jumping 10 levels.
- The smallest is B2W2, with only a 1 level difference.
- You can easily see where a lot of story takes place in each game by looking for large level gaps between Gyms.