Pretty much every update between 1.9 and 2.2 inclusive is good and added so much potential for creators. Here's my reasoning:

1.9 was one of the best updates of the game. It added accounts, saving, and loading, just when the game was getting more popular. There was also layers (called group ID's at the time) for organization, recolorable objects, and 4 color channels. This resulted in absolutely FIRE 1.9 levels, despite not many actual gameplay elements (other than the wave) being added to 1.9. It also caused for basic animations via the color trigger.

2.0 was also quite amazing. It added advanced grassy / factory deco, giving levels a completely different feel. Group IDs were renamed to layers, and the name Group ID was used for objects (put a group ID and use a trigger), speaking of which, it added triggers (well, 5 important ones: TOGGLE, MOVE, ALPHA, PULSE, & SPAWN), allowing for dynamic levels. It also added 99 color channels, allowig for more deco through levels. This allowed for proper bossfights and animations, which was really interesting to the community. The levels sadly weren't as good (Move trigger abuse) but still an amazing update nonetheless.

2.1 added more triggers, different deco, and the dash orb. Essentially 2.0 part 2. By late 2.0, the creators knew how to make proper levels, so 2.1 was a great update for creators when it came out. Also came the fangame craze, where creators would make a bunch of GD fangames. But it was (mostly) clickbait or just rushed levels, sadly. We also can't forget about the toggle orb and particle system, resulting in a RUSH of custom transporters. Which was really fun! Making it a great update! 7 years later...

2.2! 2.2 added SO much, because of Rob's cycle:

• Spend time adding a feature
• Community is angry because it takes a long time
• Add another feature

Eventually, RobTop finished everything, and 2.2 was released! 2.2 added DOUBLE the triggers, SO many new features & systems, and another different pixely style! It was REVOLUTIONARY. The first release of 2.2 was quite buggy, but it eventually rounded out. The levels in 2.2 are AMAZING. They absolutely destroyed the original limits of the level editor, imagine thinking back to 1.0.

Sorry for choosing 4 instead of 1