A quick search on GameMaps[www.gamemaps.com] or Steam Workshop shows hundreds of custom campaigns, and in this guide I selected those that can be considered as the Top 50.
The campaigns were evaluated according to three criteria: graphics (detail of 3D models, textures, design, lighting, etc), originality and gameplay on expert realism. The presence of unsolved bugs, crashes or conflicts was also taken into account.
The degree of difficulty of a campaign (its events and the frequency of spawning hordes and special/common infected) does not interfere in its gameplay note, because this basically depends on team that is playing, its tactical capacity (teamwork) and the difficulty level of the game. We must not confuse difficulty (depends on team) with impossibility (depends on map). All campaigns in this guide were played/finalised on expert realism, so there is no impossibility.
Furthermore, from the Top 50 I selected the 15 best campaigns (the Top 15 list, with ambient screenshots), and put more suggestions of good and playable custom campaigns/maps.
This guide will be revised and updated constantly.

The campaigns here are directed to GameMaps[www.gamemaps.com], but you can also look for them on Steam Workshop. To install via GameMaps, go to the campaign page, download the file and unzip it. Then copy the VPK to the addons folder of Left 4 Dead 2, which is normally located in the following path: C:\Program files (x86)\Steam\steamapps\common\left 4 dead 2\left4dead2\addons.
Notes:

1. To facilitate the organization of the VPKs, create a shortcut on the desktop to the path: C:\Program files (x86)\Steam\steamapps\common\left 4 dead 2\left4dead2\addons.
   
2. When installing a new version of a campaign/map, first delete the old version.
   
3. Keep only a few add-ons (campaigns/maps/mods) installed at the same time to avoid conflicts (you can also enable/disable them through the "Add-ons" option of the game's main menu).
   
4. If errors persist, remove all custom content except the campaign/map you will play.
   
5. In the case of a local server, if you are having consistency errors (map string errors), there must be differences between the server files and the files of a player. To correct such errors, the local host should open the console and write "sv_consistency 0" (without quotes). And, of course, make sure that all players have the same version of a campaign (as a general rule, allways use VPKs from the same source). For non-local servers, update your campaign to server's version (if necessary) and follow step 4 above.
   
6. Don't forget to read the section "Valve's Missing Content Fix " of this guide.

Important: there is no order of preference in this list!

Fall in Death[www.gamemaps.com] (Top15)
Vienna Calling 1[www.gamemaps.com]
Vienna Calling 2[www.gamemaps.com] (starts exactly where "Vienna Calling 1" ends)
Heaven Can Wait II[www.gamemaps.com] (Top15)
Heaven Can Wait HardVersion[www.gamemaps.com] (hard version)
Dead City II[www.gamemaps.com]
Dead City II Lite[www.gamemaps.com] (short version)
Warcelona[www.gamemaps.com] (Top15)
One 4 Nine[www.gamemaps.com] (Top15)
Death Aboard 2[www.gamemaps.com]
Suicide Blitz 2[www.gamemaps.com]
I Hate Mountains 2[www.gamemaps.com] (Top15)
Dead Series[www.gamemaps.com]
Dead Series Original[www.gamemaps.com] (first version)
Blood Tracks[www.gamemaps.com]
Back to School[www.gamemaps.com]
Redemption II[www.gamemaps.com] (Top15)
Redemption Hazmat Charger[www.gamemaps.com] (suggestion)
Precinct 84[www.gamemaps.com]
Left Behind[www.gamemaps.com] (Top15)
Diescraper Redux[www.gamemaps.com]
City 17[www.gamemaps.com] (Top15)
Symbyosys[www.gamemaps.com] (Top15)
Symbyosystexteuresfull[www.gamemaps.com] (install this VPK too)
Fatal Freight[www.gamemaps.com] (Top15)
Undead Zone[www.gamemaps.com]
Highway to Hell[www.gamemaps.com]
Highway to Hell Again[www.gamemaps.com] (more stable version)
The Bloody Moors[www.gamemaps.com] (Top15)
Dark Wood[www.gamemaps.com] (Top15)
Farewell Chenming[www.gamemaps.com]
Energy Crisis[www.gamemaps.com]
Dead Before Dawn DC[www.gamemaps.com] (Top15)
Dead Before Dawn Extended[www.gamemaps.com]
Tour of Terror[www.gamemaps.com]
SaltHell Park[www.gamemaps.com]
Drop Dead Gorges[www.gamemaps.com]
Escape From Valencia[www.gamemaps.com]
Urban Flight[www.gamemaps.com]
Journey to Splash Mountain[www.gamemaps.com] (Top15)
Night Terror[www.gamemaps.com] (Top15)
Night Terror (L4D2) Custom Sounds Fix[www.gamemaps.com] (install this VPK too)
Haunted Forest[www.gamemaps.com]
Last Summer[www.gamemaps.com]
Last Summer v5[www.gamemaps.com] (map with missing textures included too)
Yama[www.gamemaps.com]
Dead Vacation[www.gamemaps.com]
Detour Ahead[www.gamemaps.com]
Questionable Ethics[www.gamemaps.com]
Questionable Ethics: Alpha Test[www.gamemaps.com] (sequel to Questionable Ethics)
Cure 2[www.gamemaps.com]
Absolute Zero Complete[www.gamemaps.com]
Dead Echo 2[www.gamemaps.com]
Deadenator (for now, only on Steam Workshop)
Silent Hill: Otherside of Life[www.gamemaps.com]
Dniepr[www.gamemaps.com]
RMS Titanic[www.gamemaps.com]
Blight Path[www.gamemaps.com]
White Forest[www.gamemaps.com]
Day Break[www.gamemaps.com]

Important: there is no order of preference in this list!
From the Top 50 Custom Campaigns, it's possible to select those that can be considered as the Top 15. In these campaigns, the three criteria: graphics, originality and gameplay are maximum.
Ambient screenshots were added in this session.

More suggestions of good and playable custom campaigns.

More suggestions of good and playable single map custom campaigns.

More suggestions of good and playable modified official campaigns.

"Valve's missing content fix" adds missing textures and models in some custom campaigns (listed in the download link below). These files came in with the first revision of Cold Stream, and the reason they have been removed is because Valve had to remove some content that was unsourced when they released Cold Stream. This happened on July 2nd 2013.
TIP: Always keep the file "missing content.vpk" in the addons folder of Left 4 Dead 2.

Download: Valve's Missing Content Fix[www.gamemaps.com]
Created by Rayman1103

http://steamproxy.net/sharedfiles/filedetails/?id=625654184
This guide was written by me, GothMinister.
It reflects my opinion after thousands of hours playing Left 4 Dead 2 (and hundreds of hours of Left 4 Dead 1 and the first custom campaigns like Dead City, Death Aboard, Heaven Can Wait, etc).
I love this game for many reasons, but mainly because of the quality and quantity of available custom campaigns, many of them even better than the official ones. Thus, I decided to create this list of the Top 50 as a reference for the best custom campaigns to play.
Suggestions and comments are welcome.