BO2 multiplayer was the worst in the series. At it's core BO2 is broken and unbalanced. It was designed for competitive gamers and yet is possibly the least skillful in the series. The lag compensation built into matchmaking completely defeats the competitive focus and yet they forced SBMM into pubs. Talk about dreadful.
I enjoyed the flexibility over BO I. I don't play much online otherwise.
As for single player, the only redeeming quality was that you could customize your class before each mission. Both Black Ops campaigns imo are boring to play, but Black Ops 1 had the better story. Having three endings is an interesting decision, but the writing was bad so why would care about the endings. To me the Modern Warfare series had better characters and story. All Infinity Ward games, even Ghosts had better gameplay and setpieces. I only play through campaign 1x on veteran. I'm not a huge fan of any in the series because I never bought COD for campaign...even COD 2 which was my first Call of Duty.
I never bought CoD for the campaign either, but I enjoyed the campaign in BO/II. Let's just agree to disagree.
As for zombies, BO2 zombies had a garbage storyline, but maps like Die Rise, Mob of the Dead, and Origins are improvements to the series gameplay wise. I'm not a huge zombies fan, but I do get to round 50-100 at least once on each map. To me the main problem with zombies is the overall lack of maps. I just can't justify playing one map for three months. That's why I think the custom zombies community is so big on youtube, but I'm not a big PC gamer so I never invested into it. However, I can't see how maps like Nacht or Der Riese are better than Die Rise and Origins. I guess I can understand if the atmosphere and story are that big of a part of the gameplay to you.
Never bought any of the DLC for BO II, zombies was my biggest motivator for the DLC in BO I, so after that, I didn't want to buy them.
Black Ops 1 in my opinion was the only good Treyarch game. It wasn't free of problems, but it had variety, balance, and a decent fun factor. COD 3 was meh, COD 5 was meh, but Black Ops 2 probably killed the series. Let's be honest, MW3's major problem was lag compensation. Gameplay wise it was still a pretty fun game. Black Ops 2 sucked the fun out of COD and kept the broken lag compensation.
There's a lot of factors why COD is declining. I would argue netcode issues and Black Ops 2 are the biggest. I think Black Ops 2 changed the community in a very bad way. The competitive scene is overreaching and ruining the game for the casual audience which make up the majority of COD players. People also got tired of the bad lag compenstion and they no longer even see a fair game when they see it. People got so used to an unfair game like BO2 that they complained with Ghosts actually got netcode right. Ghosts actually has one of the lowest autoaims in the series, but there was still a percieved increase in the TTK because lag compensation was fixed. In reality, the gunplay in Ghosts was in line with any Infinity Ward game. There was no increase in TTK
As for Black Ops 3 selling big it depends on a lot of things. Did Treyarch learn from BO2? Are they going to continue to alienate the majority by putting in SBMM in pubs and forcing competitive down our throats? Are they going to launch with broken netcode that renders each gunfight pointless? Is the game going to have perk balance? Are snipers going to get special treatment? What time period will this game take place?
Treyarch's game will probably outsell Ghosts and Advanced Warfare. The console transition alone has contributed to some of the drop off. Zombies will provide a small boost, and Treyarch's game will probably be in a familiar timeline rather than a futuristic one. If it's a future game like BO2 then it's going to lose against Star Wars Battlefront and Halo 5. I don't see it selling BO2 units. To me COD will never get numbers like that again unless under one of the following scenerios: a free to play remix of classics (like the one in China), a new modern warfare, or a completely new engine that shatters our expectations (which is the least likely). We are proably several years off a new Modern Warfare. This gives people like Zampella plenty of time to develop their own modern FPS providing that Respawn gives up the Titanfall brand. The future of COD is anything but certain.
I can see you are pretty invested in CoD. I am not, so it doesn't matter too much to me.