Well they all of their strong points. What do you want from a phone. I am an OPO owner and have been very happy with my phone, but I have also ready that the 5X and 6P are amazing phones for the price.
ARS had some great reads on the phones you mention:
http://arstechnica.com/gadgets/2015...s-the-wrong-corners-to-stand-out-on-a-budget/
http://arstechnica.com/gadgets/2015...-the-true-flagships-of-the-android-ecosystem/
The OP2 is better than it was at the time of this posted review, but still all pros and cons for each need to be analyzed. You should spend time researching this, and also detail what you want and expect from your next phone. The more you can provide about what you want the more fine tuned answers you can get.
If I had to choose now, I'd consider the 5X because I'd like the smaller size while still having strong features and a competitive price. But the 6P is tough to ignore, and so is the OP2. The OPX just came out too, it's a OPO reworked into a smaller 5" screen form factor with a smaller battery and the OxygenOS. But initial reviews says the OLED screen isn't as good and the camera sucks (some of which is software issues). But it has a much more attractive price and an SD card slot. You could buy two of them with your budget in the US at least...hopefully their price is around the same in your area.
