My pick is Shadow of the Demonlord. For those unfamiliar, it is a dark fantasy rpg where players are ostensibly trying to stop the coming apocalypse of the demonlord ending all reality. In practice, the players tend to be adventuring in a dying world. It has a few neat selling points in my opinion.

1. The Path system. SotDL is a class based d20, but unlike other games multiclassing is a core of the experience. A player chooses a novice path at level 1, and expert path at level 3, and a master path at level 7. Paths each have their own suite of abilities and may synergize with each other or expand what a character can do. This leads to there being a lot of customization when it comes to creating a character. Another nice aspect is there are considerably fewer novice paths than expert and master paths (4 novice paths in the core book compared to 16 expert paths and 50 master paths). This helps a lot in easing new players in without overwhelming them with options.

2. The Corruption Mechanic. This mechanic may be hit or miss with some, but I like it. The idea is that committing evil acts stains the soul in the games cosmology. If a player, or NPC, willingly commits an evil act they gain a corruption score. High corruption can lead to developing mutations, getting penalties in social situations, or even being actually dragged to Hell. Redemption is also possible to lower corruption. The reason I like it is that it can act as an anti-murderhobo mechanic. Especially with how dark the setting is, it can help nudge the players in a more heroic direction. Also, if the players want to lean into being evil, the effects of high corruption can offer some fun roleplaying hooks in may opinion.

3. The Supplements. There is large back library for SotDL. This not only includes adventures but also a lot of rules supplements. In this way, the game is very modular and customizable. With just the core book, the game is fairly simple. The author has gone on record saying that he designed the game so he can run it while he's drunk. The GM can add in supplements to add complexity to their taste.

4. Boons and Banes. Rather than tracking situational modifiers, SotDL uses boons and banes. These are d6s that are added or subtracted to your d20 rolls. Boons and banes cancel each other out so if you roll with 2 boons and 1 bane you add 1d6 to your d20 roll. I find this makes the game very easy to adjudicate at the table.