I'm big on OU for a couple reasons (not the favorite mon part. most of my favorites are bad ):
First, I do vibe the most with the power level; I really enjoy piloting highly proactive and uniquely powerful Pokemon as it can be really rewarding in some contexts, or just satisfying in others.
The second and main reason actually comes down to the role compression seen in OU-level Pokemon. There are many top-tier Pokemon that gain insane usage through their ability to be splashable and blanket check many things; think Landorus-T, Zapdos, Great Tusk, etc. Because so many OU-caliber Pokemon can fundamentally be good glues by proxy of a great all-around profile, it's far easier for me as a builder to experiment with unexplored cores and Pokemon in OU where I can fill the necessities of a well-rounded team by having a few insanely reliable glue picks that can cover me to some degree in most matchups and leverage some experimentation. Top-tier glues can and do exist in any tier, but in OU, the power level is generally more dynamic and there are holistically more options to cover different weaknesses depending on what you're experimenting with.
To show you what I mean, and why I love OU over other tiers, here's an example of a team I built, and how/why I built it the way I did:
This is a team where I wanted to experiment with SD Sneasler and SD Rillaboom on the same team, since SD Rillaboom can be an absurdly good mid-game breaker that just so happens to help overwhelm Sneasler's most common checks. I also wanted to use Sash Ceruledge on this team since it, too, can be a great midgame breaker that also aids in the Clefable, Iron Moth, and Iron Valiant matchups, and AV Samurott is also a treat to use (and I wanted to mess with it too now that it has Flip Turn).
Zapdos and Great Tusk allow me to make a fun yet interesting team like this more freely because they just cover everything to some extent, and make this kind of structure able to actually do something at a higher elo. I've been hovering around 1800-1900 with this team while it was still being tinkered with. For reference: Great Tusk is removal, a decent offensive threat that blanket checks most physical threats of the tier, and Zapdos's all-around great defensive typing covers me into most things in general while granting me a good Special attacker, easy status spreading, and offensive pivoting. These two Pokemon are amazing and are glues that bind this team together in a way that makes it actually able to work because they're able to generally do a lot that's necessary to make a consistent team.
TL;DR: OU's power level is just ripe enough for me to enjoy more easily, but that power level in addition to being enjoyable to mess around in, leverages more interesting teambuilding and tier experimentation/growth by providing the format with higher-quality glues.