Well all those are premium smartphones, so its sure that each of them has a unique blast of their own! As you have asked which one of them is best in each specs category, so here I will provide you to the point answers.
DisplayApple iPhone 6 has 4.7 inches of display followed by 5 inches of HTC One M9. Whereas Samsung Galaxy 6 and LG G4, both features same resolution pixels, i.e, 1440x2560 pixels. But LG G4 has the biggest screen among them which is of 5.5 inches and Galaxy 6 has a screen display of 5.1 inches. Hence the clearest display will be of Samsung Galaxy S6.
Winner: Samsung Galaxy S6 is the winner due to its more PPI.
CameraApple iPhone 6 has the 8MP of rear cam and 1.2MP front cam which is the lowest among these smartphones. Galaxy 6 has 16MP of rear cam and 5MP of front cam. But LG G4 camera wins over Galaxy S6 as LG G4 has 16 MP of rear cam and 8 MP of front cam. And greatest of all in camera specs was found to be of HTC One M9 as it has 20 MP of rear cam and 4 Ultrapixel front cam.
Winner:
HTC One M9 wins here with its whooping 4 ultrapixel front cam as well as 20 MP of rear cam.
RAM and StorageApple iPhone 6 has only 1 GB of RAM and 16 GB internal storage and amazingly Samsung Galaxy S6, LG G4 and HTC One M9, all of them features 3 GB of RAM and 32 GB of internal storage.
Winner:
Samsung Galaxy S6, LG G4 and HTC One M9 are winners here.
Processor and Gaming ExperienceiPhone 6 features A8 processor and LG G4 has 1.8GHz hexa-core Qualcomm Snapdragon 808 processor. HTC One M9 and Galaxy S6, both have 1.5GHz octa-core processor but their processor make's are different. Samsung Galaxy S6 processor make is Samsung Exynos 7420 whereas HTC One M9 processor make is Qualcomm Snapdragon 810.
Winner: HTC One M9 wins and will definitely provide you a good gaming experience with its 1.5GHz octa-core Qualcomm Snapdragon 810 processor.
Battery PerformanceHere, Apple's iPhone 6 features lowest battery life with its only 1810 mAh non-removable battery. Galaxy S6 has a battery life with its only 2550 mAh non-removable battery followed by a bigger battery life of 2840 mAh non removable battery of HTC One M9. But the biggest battery life is of LG G4 as it has 3000 mAh removable battery life.
Winner:
LG G4 is the clear winner here with its 3000 mAh battery life.
I have tried to cover all of the aspects of the smartphones which you mentioned, plus I have provided you the clear winner of each specs category which will make it easier for you to choose the best one from them.