Say what you want about the Sinnoh games as a whole but Sinnoh, as a region, is fucking fantastic.
It's got a reasonable handful of unique biomes yet explicitly keeps them all believably tied into a northern climate.
It has a consistency in its overall atmosphere that keeps it all feeling distinct and memorable as a cohesive whole.
The way in which you progress through it is also great in my opinion.
It's the perfect mix of linear and non-linear, with just enough backtracking to make the region feel like a sandbox you're uncovering and settling into rather than a series of linear areas you're being guided through, yet not so much backtracking that your progression feels senseless.
All the complaints and criticisms Sinnoh faces are inflated beyond their actual proportions, too.
HMs only start to become excessive late game and by then you'd ought to have a lot of pokemon so it's not an issue.
Things like the snow and mud are minorly annoying at worst, and they increase your overall playtime by like, MAYBE 5 minutes, most of your time will be spent battling anyway.
The regional dex is a touch small in DP but it's not remotely hard to put together a good team unless you explicitly apply your own autistic restrictions on what can and can't be used.
The fact that these are the best criticisms people can come up with is really a testament to Sinnoh's overall quality.