Russian Overkill is not a mod for someone who wants a challenging, balanced Doom experience. However if you like blowing shit up with insanely overpowered weapons and seeing demon gore fly across your monitor, then Russian Overkill is probably your best bet. Russian Overkill adds 41 different weapons in all categories, including melee weapons. Even the melee weapons the mod adds are insane—a stick grenade that you smash into enemies heads and watch them explode into gory bits, or the Excalibat, a baseball bat that launches energy bolts.
Most of the weapons that Russian Overkill adds have an alternate fire as well, which makes all of them just that much more insane. My favorite weapon in Russian Overkill is the quadruple-barreled shotgun. The alt-fire makes you sprout two extra arms so you can fire four of them at once for 16 barrels of buckshot flavored chaos. If that doesn't at least make you crack a smile, you might want to check your pulse. In terms of shotguns, there is also the Schwarzer Zwerg, a belt-fed machine gun that fires a 12 gauge payload. Also, the Pantokrator, which unleashes a ten shell payload with a single pull of the trigger and the alt-fire lets hundred of shells fly set to Bizet's Carmen Prelude.
The machine gun category is a little more boring with multiple chainguns and a couple other standard machine guns. Luckily, the larger weapon categories don't disappoint. From nuke launchers to railguns to chaingun rocket launchers, this is where Russian Overkill gets really crazy. Like I said, these weapons will make you incredibly overpowered, so I wouldn't recommend playing through anything but the hardest .wads or just turning on all weapons and going nuts. You also choose a class at the beginning of a new game. Your class affects what pistol you start with, and each class has a few special abilities. Most of the abilities are crazy melee attacks that you'll have to perform combos to complete (including a Falcon Punch that you can set loose with ADWWS Fire if you select Yuri Dubienko)
Not only will your weapons be insanely overpowered, but this mod also allows you to stack armor and health pickups infinitely. I used all weapons and just ran through DOOM II levels as fast as possible, and by the end of the 4th level I had 1600 armor and 2400 health. So grab a crazy .wad full of hundreds of enemies, turn on all weapons and let the rockets fly.