I’ll happily second Ironsworn as a great GMless game. Here are some of my reasons why:
GMless PbtA with great oracles
Ironsworn is not only a fabulous GMless PtbA game—primarily fueled by clever oracles that have never left me wanting whether playing purely solo or co-operatively in a group—but it presents an evolution to pbta mechanics that allow that GMlessness that could be applied to other pbta systems without too much effort.
Push-your-luck goal focus
Specifically, the push-your-luck progress and completion mechanic—that plays a key role in most aspects of the game—means that while it’s always beneficial to make more progress toward any goal (such as a journey, a combat, a ‘quest’-style goal, or a relationship), you can always attempt to complete a goal early (with greater risk), and even with a full progress track, you’re never guaranteed that the outcome will be only positive. Without explaining the whole mechanic, I adore that this makes room for emergent and genuinely surprising narrative tropes where you can take a risk and turn the tide to succeed despite being badly on the back foot, or thinking some task or effort is a routine, done deal only to realize there is far more to it than you expected all along.
Interweaving vast and close scope
The other nuance of the central progress mechanic encourages characters and parties to interweave various scopes of goals, mixing in pursuit of your overarching character-defining life goal (like finding redemption or reconciliation) with mid-level goals (like solving a local mystery) and more immediate goals (like reaching a neighboring settlement across a dangerous path). I’ve seen this motivate rich character arcs whether playing a campaign or even a one-shot. It’s surprisingly effective.
Simple+Deep Character Evolution
Likewise, I enjoy that it deconstructs the typical PbtA playbook into thematic asset cards that define characters’ uniqueness. You start with three cards, which may be Paths (bit of background/career), Companions (like pets or allies), Combat Talents (styles like shieldbearer or skirmisher), or Rituals. You then use xp to either acquire new asset cards, or to unlock more advanced moves offered by your existing assets. For instance, you may start with a kinsman Companion that initially guards your back, but later you can invest in that relationship such that the kinsman grows into also helping you in, say crafting or political tasks, and later grows into keeping up your spirits when the going gets rough. That evolving advancement feels compelling like a Skyrim skill tree, and offering vast character variability, while retaining supreme simplicity.
Relationships = Power
I love a lot more about this game, (especially the responsivity of the designer, the expansions in the works, and the fact that it’s free without sacrifice of production value), but one of the final bits I enjoy for GMless purposes is that it strongly encourages forging bonds with people, places, and groups in the gameworld (since having such bonds is not only the most explicit ‘win condition’ when a PC wishes to retire), but also because the bonuses from such bonds are one of the best way to improve your mechanical power in the game. Doing things on behalf of people or places you care about are far more effective.
tl;dr
Ultimately, if you like PbtA sensibilities, and you’ve maybe ever been interested in Mythic or any other GM emulator but were put off by the complexity therein, you may like Ironsworn’s elegance (and you can totally play it with a GM too if you prefer).
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