These mods will improve L4D2 in all aspects. Works great in single player and ONLINE multi player, here's how:
1. By hosting an online lobby that your friends can join
2. Joining an online lobby with everyone in it having these mods(especially the HOST) and not official servers that enforce vanilla gameplay.
All mods in this collection works perfectly fine together and great in any resolution. You can just click "Subscribe to all" and it's ready to play once downloaded EXCEPT for these three mods.

1. Competitive-Bots+ & Left 4 Bots (Improved A.I.)
Installation: Start any campaign 1 time after these mods finished downloading so that it'll generate a file in your steam folder, go to your folder Drive:\SteamLibrary\steamapps\common\Left 4 Dead 2\left4dead2\ems\left4bots\cfg, open "settings", and change the "load_convars = 1" to "load_convars = 0". That's it, you're bots are good to go!
For a detailed explanation, watch NasCoz here https://www.youtube.com/watch?v=kN5ej9PUskw

2. Blood++
Installation: Read the description https://steamproxy.com/sharedfiles/filedetails/?id=370399372.

All of the firearms are improved and not far from L4D2 universe(except the Magnum and AWP, they're magnificent AF), have no camera shake when bashing(except the pistol), no half-assed lazy bash animation, no missing texture, no bug/glitch/clipping, no hands handling the gun uncomfortably, and no bullets coming out of the flashlight or front sights, etc. Took me some time play testing a lot of guns to figure out which firearm mods are 100% compatible with the other mods in this collection. You can safely ignore the conflicting files(red fonts in add-ons) because some of them use the same file.

Conflicts stated:

1. Rayman1103's Mutation Mod & Menu UI Update with Rayman's Mod Support - No Groups, Alternative Addon Browser
"This is a version of my Menu UI Fix and Improvements compilation specifically designed to accompany Rayman1103's Mutation Mod. It includes every feature of the original addon as well as support for every custom mutation in Rayman's mod. Unfortunately because both this and Rayman's mutation mod both edit the menu, they will conflict in the addons menu. There is no way around this."

2. re2 lifeline animated healthbars & [HUD] Slim Hud - Alternative
"They are meant to be used together but have the same files that they use."

3. Ammo Reborn & Ammo Upgrades Reborn
"They will show as conflicting (because both mods share textures for the ammo models to save memory). This can be safely ignored. Both mods will work fine together."

Also, you can put this in your L4D2 launch option for better game optimization. Right click L4D2 in your steam library, click on "Properties", at the bottom of the "general" tab you'll find "Launch Options". Copy and paste these texts there.

-novid -high +mat_motion_blur_percent_of_screen_max 0 -useallavailablecores

Here's what they do:
-novid = Disables the intro video every time you launch L4D2.
-high = Sets the game on high priority so you don't have to do it in task manager.
+mat_motion_blur_percent_of_screen_max 0 = Disables motion blur.
-useallavailablecores = Self explanatory.
Lastly, don't forget to "Verify integrity of game files".

If you find this helpful, please rate & favorite so other players can find this too. It means a lot to me, thank you! :)