I don’t have experience with the zp5 3-15 or the zco 4-20 but I do have a zp5 5-25 and a zco 5-27. The zp5 glass does edge out the zco on clarity, resolution, and color, but everything else about the zco beats the zp5. The zp5 has the worst turrets I’ve ever used and it does have one foggy spot near the edge of the glass which is pretty common in these scopes. They also had tracking issues in their early days. I would definitely not pay MSRP for one they are not worth anywhere close to 3500 even in their current state. They are improving, but buying a discontinued model is going to be hit or miss on whether you get a good one or one you’ll be shipping back to Germany to get fixed.
I would not count out the atacr 4-16. It might be my all time all around favorite optic, and is definitely the nicest scope NF makes. More expensive doesn’t always mean more better. Nightforce seems to have defied physics with that optic. Its glass clarity isn't as good S&B or Minox, but it’s not far off the ZCO and is good enough for me to spot 5.56 impacts at 650+ yards. It has great low light performance, excellent mirage performance, extremely forgiving eye box, and it has by far the least top end mag optical degradation of any other scope I’ve used. It also is lighter than any of the others.
I’ve been behind TTs and honestly have never been able to see what you’re paying for over a ZCO. That being said I haven’t spent a significant amount of time behind a TT so someone who owns both would be able to speak to them more.
My biggest gripe with S&B is their mirage performance is atrocious. I’ve been behind 3 and own 1 and they’re garbage when it’s hot out. ZCO and nightforce blow them out of the water on hot days. S&B is almost unusable out west when it gets over 80 degrees and/or you’re running an uncovered suppressor.