Thorn, but only because The Last Word is gimicky and has a very particular playstyle which limits is usefulness. Yes, The Last Word can shut someone down before they have a chance to react, but its RNG dependent and fairly situational. Thorn is just a very powerful gun. As a Handcannon, it has decent stats, mixing good fire rate with decent impact and stability. As an Exotic, it can two shot kill someone with as little as 0.36 seconds of focus. The stability makes getting those two headshots fairly easy as well. So now that fast fire rate is also very powerful. More powerful than any Heavy Machine Gun. Or so the saying goes. Honestly, I don't use either. Thorn kind of sucks in PvE in my opinion, I really wasn't impressed, and the fact that it looks like a dogshit dinosaur makes me never want to use it. Being supposedly OP doesn't make me want to bend over to it either; but I really don't have first hand experience using it in PvP. I've definately been killed by it though.