what's the easiest mode to get a good rating?

It's all make-believe to some extent, as the rating is only an indicator of strength against that given rating pool.

A 1600 daily player vs a 1500 blitz player is incomparable and you'd probably find the 1500 blitz player would be a stronger player than the 1600 daily player.

But generally, below 2000 level, if the single objective is to have the highest rating in any given time control, it would probably be easiest to achieve that in Daily Games, then Rapid, then Blitz, then Bullet.

After 2000, the number of players playing Daily and even Rapid games drops off, and most of the high rated players play Blitz and Bullet, so those guys would probably have an easier time hitting 2600 blitz vs 2600 rapid due to the size of the player pool.
For example, as a 2100 rapid player, it says I am in the top 99.9% of players. It just shows how few high rated rapid players there are - to get points from!

i might try to play blitz more

Depends how fast your brain works - I suck at blitz, better with slower games

Puzzle mode seems to be way easier to get 2000+

You should learn most important principles of a chess game. That will give you a solid understanding of a chess game and you will know what to do in every position. Then, you should optimize your calculation so you don't miss tactics in your games. Your rating will skyrocket. I can help you with all of this. Message me if you are interested.

Online ratings are worth every penny.

Oh, wait, they cost nothing. I guess they're worthless.

Ignore your rating. Literally, ignore it. Focus instead on becoming a better chess player. Then your rating will naturally increase, to track your progress as a player.

If all you care about is having a 4-digit number next to your name, just play 10s or 30s bullet and premove a bunch of nonsense.  You will end up winning a bunch of games on time and pump up your rating.  You won't be any better at chess, but that is beside the point.

Study mode.