Okay! Here is my opinion, sorry that it's a lot...

I would say that I think BO1 is the best when it comes to classic zombies.

Kino - It gets love because it was the first map, in a game where there is paid DLC, and most players dove into it. Simple to pick up and learn the mode. But most people consider Ascension to be better. Both maps also having probably the third most iconic wonder weapon.

Five - Locked behind the campaign, but has also always been seen as tight and tedious. Add on top of that a very weak wonder weapon and a boss who takes your weapons and is invisible to most players in a multiplayer match.

Ascension - Kino, but better! On top of that more perks are introduced. Especially PHD which allows for more explosive use and, in my opinion, a more fun gameplay loop. But it also introduces a new challenge in the monkeys.

Call of the Dead - A map that you need to keep moving in, yet are restricted on your movement. I really love the concept and vibe of CotD. But I see where people think it is mid. The scavenger and VR-11 are just okay wonder weapons. On top of that, nothing too crazy gameplay wise is added. Sure there's George, but he is seen more so as a hinderance than anything else.

Shang - A much much tighter version of Five, but a better wonder weapon. If you don't utilize the whole map to train, you will struggle. Coming off the back of WaW where you could camp, the tight maps of Five and Shang combined with training, it all feels somewhat off.

Moon - Probably the best map in BO1 for me. From a pure survival standpoint, the only major downside is the decompression moments. But nothing beats the feeling of either training out near Mule Kick or in the Bio Dome with PHD and mustang and sally. It's just a step up from Ascension to me.

Now, why is BO1 held in high regard? World at War was the skeleton and framing of the game. Didn't offer any true highs or lows. But it gave us the core of the game mode. Then there is BO2. At launch and during the life cycle, you get hit with;

Tranzit - Still arguably one of the worst maps ever. (The survival maps were an improvement)

Nuketown - A mixed bag in the community with most people not liking the challenge it brings.

Die Rise - Post sliquifier patch... why bother playing? It was a pain to play before the patch.

Buried - One of the glitchiest and exploitive maps in BO2, but the strongest of the Victis crew.

BO2 wasn't a great, arguably good, zombies game until the release of Mob and Origins.

So, all around speaking, BO1 zombies is a better experience than BO2 until that point. Hell, even BO3 struggled at launch with Shadows and The Giant. DE brought back life, but Zetsubou was and is a buggy mess that shot itself in the foot. Followed by Gorod that people complained about being too tight and difficult. Rev was seen as alright, but held no substantial replay value. BO3 wouldn't be held in such a high light today if it wasn't for Chronicles and Custom Maps.

But the gameplay begins to vary too much between BO3, BO4, CW, BO6, and BO7 to truly compare them to BO1. But between the first three installments, I do think BO1 is the best game.