I'm not a Sonic fan, but Green Hill Zone is a masterpiece. I'm not sure if it's better but anyone trashing Green Hill Zone is trashing Sonic, not whether or not that first level works.
The very first thing you see in the level is 3 rings. If you jump at the rings, and you over shoot, you land on the TV set, immediately collecting more rings and showing how sonic jumps on those items to collect them. If you miss, you land on the baddie on the the lower level, doing the same thing. This will introduce you to Sonic's floating rebound and momentum mechanics immediately.
If you bounce on the baddie on the lower level, it bounces you up to the upper level anyway, where you are presented with another baddie. Hit or miss that baddie, you will hit the hidden spring in the tree, slamming you up to collect more coins.
This is conceptually similar to the opening of 1-1, and the rest of the level proceeds to lay out all the core mechanics of the game. Almost forcing you to get hit and see how rings protect you. Placing rounded corners right after straightaways without enemies, so you naturally run and understand how Sonic rolls. The first branching path in the game comes after a jump onto a ledge that crumbles beneath you, almost forcing players to discover that the fall actually opens up a second path, quickly showing players how these levels are solvable in multiple ways.
At the end of Green Hill you know whether or not you're gonna like 2D Sonic, That doesn't mean that you do like 2D Sonic, but you can't argue that the game puts it's best foot forward, with great visual design, popping colors, and a great tune, all things the series is known for.
I played both games not long after they came out in the states, and I found 1-1 brutally hard (I was 5). I completed Green Hill zone without a game over. Yet at the end of 1-1, I had seen all 1-1 had to offer. At the end of Green Hill, there was plenty to discover in that level (a fundamental pillar of Sonic, in fact). 2D Sonic has never left that first level behind, but plenty of 2D Mario has left 1-1 behind. I can't tell you how frustrating it was to barely get past a Goomba, heart racing, only to realize I couldn't go back and find out what was in the ? blocks.
Green HIll looks and sounds better, but that's a consequence of being on later hardware, after the 2D platformer had really crystallized around Mario. 1-1 is a little more primitive, but the gameplay is certainly more broadly popular than Sonic's. Both levels are some of the best at introducing players to the game they're about to play, teaching them through gameplay, and telling players straight up what experience they are in for.
Naoto Ohshima and Takashi Tezuka both don't get enough credit.