If you didn't already know, Treyarch allowed workshop content and produced mod tools in Call of Duty (COD) Black Ops (BO) 3. This allowed users to develop their own custom zombies mods and maps. It has added much playability for my buddies and I while waiting on developer promises for other game releases.
#1 Octagonal Ascension
This is my all time favorite map. I will highly respect anyone that makes it to the end.
Octagonal Ascension requires fast paced decision making. As any map, perseverance is key I imagine to win. Be sure to watch your footing. See below the common interaction with zombies.
These zombies are ruthless. They will stop at nothing to take out you and your friends. Octagonal Ascension is a multi floor maze-like zombies map. You must make it to the top, but don't forget there is a buyable ending. Power ups are delivered by zombies as usual and sometimes power ups are a weapon like the Ray gun or an AK. Pack a punch power up is a feature on this map, so make sure your friends are prepared with their weapon to upgrade (if you even have time to tell them!). Special zombies spawn in the early rounds. Be prepared!
What really makes this map number 1 for me is the fact you can get to end game very quickly in this map and are faced with challenges very early. I know you will really enjoy this map.
https://steamcommunity.com/sharedfiles/filedetails/?id=2354612152
#2 One Barrier Roulette
One barrier, two barrier, and three barrier maps are very common in the custom map community. This one takes the cake. It was created by Cesar and he does not hold back on this map. He incorporates social media trends and a bit of pop culture in this one and other maps he creates as well. You can check out his workshop on Steam: https://steamcommunity.com/id/mcdonaldscomingsoon.
I encourage you not to watch any videos of this map and just do it. 'Roulette' is the word to focus on. It makes this map a blast. Make it past round 25 and you will win. Enjoy!
https://steamcommunity.com/sharedfiles/filedetails/?id=3352256337
#3 Viking Tower
Viking tower is a great name for this map. Vikings have to be tough right? That is what IceGrenade, the creator of this map, expects out of you. Weave through tight spaces to retrieve your perks and remember to use your viking axe in the beginning of the game to get points early because you will need them early. There is a zombie shield available early on to unlock. At least obtain the shield before taking on the long climb up the tower. The zombies can sneak up on you fast even at round 3, believe it or not.
IceGrenade has been known to create pretty tough maps. You should check out his Steam workshop. He has many themed maps to explore. You will not be disappointed by his maps.
https://steamcommunity.com/sharedfiles/filedetails/?id=2496507828
#4 Toy Story: Andy's Room
I had to include Andy's Room because it has brought so much joy and nostalgia. I'd suggest this as the first custom map you play on BO 3 if you haven't played one already. There's plenty of things to do if you choose to play this one solo.
There is a buyable ending. It make take a few tries to get there.
Andy's Room was made by FrankCastle.
Remember to check the cracks and crevices to find all the woody toys.
https://steamcommunity.com/sharedfiles/filedetails/?id=2030610148
#5 Moon: Last Man Standing
Moon, created by Biscot, is the best versus map I have found. Each player in your party competes to be the last man standing. You are given your own area to operate in. The area contains a mystery box, mystery perk machine and sabotage options.
A special zombie spawns in each player's area every 4-5 rounds to add extra pressure.
https://steamcommunity.com/sharedfiles/filedetails/?id=3242850767
#6 Zombie Tower
The Zombie Tower map is worth a play. Take advantage of the early rounds and the game is beat. Having a friend on this one pays dividends. This map allows you to bank your cash to share with friends. Banking cash is a underrated feature in zombie maps. Tranzit introduced this to us and unfortunately it has been grossly unused in custom zombies and Treyarch zombies.
At the top of the tower you are awarded with necessities. Do not let that temptation deter you from getting ample points to enable you to use these things when it opens.
There's a lot of neat nuances with this map and that being said, it is a must play.
https://steamcommunity.com/sharedfiles/filedetails/?id=798845430
Bonus! - Timed Gameplay Mod
As a bonus to you who made it to the end of this page: Recently someone has released something called the "Timed Gameplay" mod. It allows any map, including original maps to be played at a very difficult pace. Zombies continuously spawn, rated for whichever round is displayed on the screen. After a certain time has elapsed, the round is advanced and new zombies will spawn at that round's rated zombie health. Some of these maps presented in this article already have timed gameplay enabled. See MystifiedTulip on Steam for more new useful mods.
https://steamcommunity.com/sharedfiles/filedetails/?id=3485963522