2015 one. And it's a lot faster than a new one.
I don't think it is quite so black and white.
http://wccftech.com/intel-skylake-cpu-10-20-performance-boost-faster-igpu-ddr4-overclocked/
Which is faster;
2015' MacBook Pro 13' Retina Display: i5 2.9GHz Broadwell CPU.
vs.
2016' Macbook Pro 13' non Touch Bar: i5 2.0GHz Skylake CPU.
It seems to me that there is definitely some gray area here. I.e. a 2.0Ghz Broadwell is Probably quite a bit slower than a 2.0Ghz Skylake, I honestly don't know enough about it to really comment but would love more insight if anyone does.
This site has a lot on it:
Skylake:
-14nm vs 22nm process, causes a reduction in power usage, especially on lower-clocked CPUs. E.g. i5 6600 = 65w TDP, i5 4690 = 84w TDP.
-Skylake has a much bigger, more modern iGPU. If you're not using a discrete card, it's much more useful vs Haswell's.
-Skylake iGPU has hardware acceleration for decoding and encoding of more formats.
-Skylake runs cooler, for whatever reason, with similar maximum safe temperatures, which is probably a factor in why it has more overclocking headroom (on average) vs Haswell.
-Skylake is 5-10% faster per clock, due to an updated core design.
-Skylake has a much wider core, which benefits more from hyperthreading. A Skylake i3 is more faster (yes, that's what I meant to say) than a Haswell i3 than a Skylake i5 vs Haswell i5. For this reason, the i3 6100 is quite dramatically better than i3 4xxx.
-Skylake uses DDR4, which is faster than DDR3, and rapidly dropping in price. Before long it will be cheaper than DDR3, too.
http://www.tomshardware.com/answers/id-3061164/relevance-broadwell-skylake-haswell.html