First thought was my old vampiric broadsword from Angband. It doesn't have the bisection capabilities of the Katana of Groo but every non-fatal blow that draws blood with it is followed by a transfer of health from victim to wielder, also known as a transfer of wounds from me to the monster I just wounded. It's pretty much designed for taking on a thousand ogres in a narrow corridor.
Then I read the OP and the bit about "from your world" and my first thought is the hybrid L86/L98 with the long, heavy barrel and bipod and the manual action because the manual action has never jammed on me and the gas action always produced stoppages.
With some more thought, the silenced version of the AK74 or its 100-series Black Kalashnikov equivalent came up as a desirable alternative.
For a video-game weapon, the bizarrely inaudible .50 sniper rifle from HALO CE is tempting especially with bottomless magazine rather than infinite reloads but I think the four-barrelled nailgun from original Quake would probably be better. It didn't quite have the obliterating power of the Thunderbolt but it did well enough and it had a lot more range.
For style points, a long spear. [http://www.coldsteel-uk.com/store/Assegai.html] I'd have said quarterstaff but those things have more terrain limitations.
Are these monsters at all smart?