The Grunt in Halo 5 on Genisis minding his own business on the ledge, enjoying the view and humming his catchy tunes to himself and playing with some cool human skull he stripped back the flesh from probably for lunch, until his untimely demise by way of being punted into the depths below by our favourite hero.
But in seriousness. To chose one less spoken of. Kurt-051, or Kurt Trevelyan, detonating two nukes to protect Blue Team, his Spartan III's, Halsey, Mendez and for asset denial of the Forerunner shield world that was within Onyx.
His self-sacrifice, by way of flow on effect, destroyed an entire Covenant fleet and saved the most valuable UNSC assets to date (note including John). And thanks to that, various technologies were discovered that went into finishing and upgrading the Infinity and helping to secure UNSC dominance over alien threats.