The best bang for your buck would be an i5. Unless you do a lot of batch processes that might benefit from hyper threading or use custom filters that behave a bit different, most of what photoshop does is single threaded. If you don't want to get into overclocking, consider an i5 6500. Clock speed will improve performance across all tasks so the 6600k with a z series board and decent aftermarket cooler may not be a bad idea.
Another way to go would be the 6700k since it's clocked a bit faster out of the box but an i7 does cost more for the cpu. It won't be as fast in terms of clock speed as an oc'd 6600k for about the same price. To get the extra 300mhz out of the box from the i7 6700k you'd still need a cooler (k series don't ship with any cooler). The i5 6600k + cooler + z motherboard work out to about the same cost as a locked i7 6700 + motherboard however the clock speeds of the i5 oc'd could easily result in 500-600mhz faster.
An i7 would be the 'best' given it will have a small advantage in situations where multithreading can be used but the cpu itself costs around 50% more. By the time you get a xeon with a fast clock speed the cost would be pretty high.
Here's some testing with extra cores in photoshop. Keep in mind the i7 6700k benefits from hyper threading over an i5 though, not additional cores. They both have 4 physical cores and differences here are noted from using 10 core xeons with various amounts of physical cores disabled. Hyper threading makes less of an impact on performance than gaining additional physical cores.
https://www.pugetsystems.com/labs/articles/Adobe-Photoshop-CC-Multi-Core-Performance-625/
For lightroom.
https://www.pugetsystems.com/labs/articles/Adobe-Lightroom-CC-6-Multi-Core-Performance-649/
There are some places that have done what they call a 'custom workload' for photoshop but I'm not seeing what the 'custom workload' actually is. It makes me wonder though since in this review techspot claims the i7 4790 does the same work in 16.7 seconds that takes the i5 4690 25.3 seconds.
http://www.techspot.com/review/972-intel-core-i3-vs-i5-vs-i7/
However in this 'custom workload' for photoshop they show an i7 4790k (which is 400mhz faster than the 4790 out of the box) completing the task in 14.4 seconds and the i5 4690k (clocked exactly the same as the 4690 out of the box) taking 20.4 seconds. So apparently when you speed the i7 up by 400mhz and pit against the same i5 the time to finish the test went from almost a 10 second spread to a 5 second spread. Something iffy there.
http://www.techspot.com/review/1041-intel-core-i7-6700k-skylake/page7.html
Meaning I would tend to question what 'custom workload' is and how accurately it represents the sort of performance difference the i7 has over the i5 by their conflicting test results. Obviously in the second test they updated it with results from skylake over haswell but those numbers I took from their tests are comparing haswell to haswell from two different sets of benchmarks results for what appears to be the same test. Given that photoshop rarely uses more than 4 cores and clock speed matters, for the cost I'd say the i5 6600k oc'd.
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