Tom's guide posted an interesting review that compares the OLED display of the iPhone X (5.8" 1125x2436) to the OLED displays of the Galaxy Note 8 (6.3" 2960x1440) and the Google Pixel 2 XL (LGD 6" 2960x1440 pOLED). This is Apple's first OLED phone, but Tom's Guide finds it superior to the OLEDs used by the other phones, especially if you want a realistic color reproduction.
The review shows how the iPhone is brighter than the other displays (574 nits, compared to 438 nits no the Pixel 2 XL and 408 nits on the Note 8. This test was performed for full-screen content. The Note 8 can actually achieve 1,240 nits but on specialized conditions and only when a small part of the screen is active.
In terms of color saturation, the iPhone display comes in between the Pixel and the Note 8. But Samsung offers several screen modes (its Adaptive Display), so I am not sure about this comparison, it seems to me that it is better to offer users their choice of color saturation mode. Two other advantages of the iPhone displays are its better white balance point and its superior viewing angles.
Regarding the display resolution, the Pixel 2 XL offers a PPI of 538, the Note 8 521 PPI and the iPhone X only 438 PPI. Tom's Guide says that you if you squint, you can see fine aliasing around text and UI elements in the iPhone that is harder to pin down on Samsung's device.