Any weapon is fine, but vehicles don't count! The relic crozius, wielded by Mordred and Grimaldus, is my favorite easily accessible xenos killer! The below image is not of a Black Templar, but I love the glorious crozius nonetheless. The barbed rounds from the chaos weaponry on Menazoid Epsilon (gaunt's ghosts the founding) claimed especially brutal fatalities to get some mention here.
Prince Yriel's Spear of Twilight. Said to house the baleful energies of a dying star. The Spear of Twilight is powerful enough to kill a Hive Tyrant in a single blow, and is so powerful that it gradually burns away the soul of it's wielder. Secondly, also wielded by Yriel, is the Eye of Wrath, which is basically a monocle that shoots lightning storms. And if that's too 'Eldar' for you Mon-Keigh, I also really like Abaddon's Sword Drach'Nyen, The "Splinter in Reality", as well as his Talon of Horus.
It's hard to beat the "Shokk Attack gun" It hovers up angry squigs or gretchin or whatever you can get your hands on and then when fired it teleports them into the warp where they become messed up and even angrier and then they're transported inside the armor (or the skin!) of your enemy! Orks have the best tech
