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. Camera
Apple 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 Storage
Apple 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 Experience
iPhone 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 Performance
Here, 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.