For me it has to be AmberELEC.
Video previews, the option to view a game guide, and an icon on the games list to indicate a game has Retro Achievements combined with an easy to view list of achievements.
It was fantastic and the reason I’m hesitant to sell my RG351V
That’s kind of how I feel about my original Xbox. The disc drives have a habit of failing and buying a new Xbox is cheaper than just replacing the disc drive but thankfully there are ways with custom firmware to replace the hard drive and play your backups.