Bo1 maps probably got the best effort. IMO
WaW not as much, the custom map tools is what made it.
BO2 is a absolute fail, i dont have much to say about this at all but that is only my opinion. The maps are grate in size and very appealing, the game play and strategies on the other hand gone south.
Yeah, I'd say Bo1 got the most attention, just because there was so many new things (weapons, perks etc)
Nacht, Verruckt and Shi no numa are all re-jigged campaign maps, whereas der-reise is a totally new map.
So my list would be:
As for bo2, I think buried had the most work put into it. With the giant, witches, permaperks, timebomb etc, lots of stuff was thought about. Even the design of the map, everything is different. On Origins, lots of the map is the same.