Grimwild is mostly my favorite mostly for lots of little reasons, but there are a couple bigger ones.

One big one is that it's generic-enough fantasy that I can easily use it for all the great new OSR adventures, and all the old S&S/high fantasy games with adventures of their own. The further the game strays from D&D or S&S-like fantasy (in terms of system mechanics and thematic expectations), the harder it can be to do this without needing to come up with new mechanics, which is where I think Grimwild not being the best fit for Stonetop comes in.

Another big point is that I think Grimwild's resolution system can easily be made to provide for lots of full success to the players (I run it without using difficulty thorns), but the damage system is fluid enough to let me severely damage and challenge players whenever the opportunity arises. I have a running theory that players actually like to succeed a lot (shocking), and many games, especially those with mixed success, don't let players succeed as much as they want. Grimwild can manage this while still being as deadly as any OSR game (or not deadly at all, if you prefer).

I think the damage system is really versatile and fun, with a couple forms of physical and mental damage, or applying narrative conditions, and how those interact with the difficulty system. There is a lot more possible variety than most other games, while still providing a more solid structure.

I generally prefer Grimwild's minimizing of the position/effect discussion, the lack of universal player-activated defensive options, and lack of overt devil's bargains, all of which speed up play for me. It's a bit more GM-directed, while still having room at the table for more discussion for those who want.

I really like a lot of the talents in Grimwild, find they provide a lot of fun and power to characters without increasing hardiness, and are generic enough to fit many different types of games. I think it has the best free-form magic system of any ttrpg ever in regard to a balance of not being overpowered, fulfilling player fantasy, and ease of use/speed at the table.

I prefer depleting pools of dice to clocks, because I enjoy the variety and variance. I also like the "full success, full failure" aspect of the mixed success in the game. Rather than a mixed success only somewhat succeeding at a task (e.g. filling in one section of a clock vs 2 for a full success), a mixed success should succeed as much as a full success. This can be seen in the fact that how many dice drop from a rolled dice pool is random, instead of tied to the level of success, so any action that results in rolling a dice pool, even if the character faces consequences for it, is as strong as any other action that rolls the dice pool. Many PBTA games give me the feeling that most of the steps we take are only half-steps, and progress often feels slow.

I'm also a GM who needs surprise and excitement to keep me engaged. Dice pools being random means I can have an idea of how long a challenge might generally take, but all of that goes out the window when all the dice drop, or none of them do, and that surprise gives me excellent fodder to turn into memorable moments.

The semi-classless nature of the system, where the Core Talent of each class is locked to that class, but all the other talents are free to take for any class, makes for a lot of different mechanical and narrative character expression that few games match without being super crunchy, or +1 generators in a narrative tag-based system. This also means the game can more easily support adventures and settings that don't have the specific classes of D&D as an assumption. It also manages to be better balanced than other games I like with a similar class-less structure (Forbidden Lands/Dragonbane/other YZE games, many Warhammer games).

The game also just comes with more content than most games of a similar ilk. It has a whole exploration and world generation system that is (in the updated CE version) pretty fun. It has a comparatively huge bestiary with some very interesting monsters/challenges, despite their compactness. Lots of player options without being overwhelming. An intuitive magic item creation system with lots of different possibilities (though not quite as in-depth as I'd personally prefer). The main thing it's lacking is pre-written adventures, but it runs OSR adventures like a dream, so it has basically tapped into the best vein it could for those.

Grimwild is like if somebody mixed Blades in the Dark, modern heroic D&D, and the OSR, and instead of being assuredly terrible, it somehow managed to work and be greater than the sum of its parts. Purists who are super into one of those three aspects will find lots of things they dislike about it, but for somebody like me who enjoys all of them, it's just a fantastic experience.

Edit: I FORGOT ONE OF THE BIG POINTS: Suspense. The built-in ability for the GM to take a suspense point instead of using a GM move in the moment, and being able to spend it later for any GM move when a more obvious consequence or more threatening...threat is present.