Which is the best smartphone between LG G4 vs Apple iPhone 6 vs Samsung Galaxy S6 vs HTC One M9 ?

Design
The LG G4 has a largest screen among all of them at 5.5 inch at qHD resolution. Besides it is comparatively lightweight at 155 grams in comparison with its screen.  HTC One M9 has a 5 inch screen and weighs on heavier side at 157 grams. The iPhone 6 is lightest of them at 129 grams, but has a modest 4.7 inch screen.  Galaxy S6 is the best balanced one with a 5.1 inch screen and a weight of 129 grams.  All of them carry screen protection. The iPhone 6 is made of metal body and the S6 has a combination of metal and glass. G4 goes most premium looking with hand crafted leather design. HTC One M9 has metal designs. Look wise,  Galaxy S6 is  attractive of the lot.

Display
As said before, LG G4 has the biggest screen of them all. It has the Quantum IPS display being the first device to employ the technology. Galaxy S6 has the same Q HD display as on G4. The iPhone 6 has the smallest with 1334x750 Retina display and 4.7 inch. Each of them have their own strengths and weaknesses. HTC One M9 has poor viewing angles while iPhone 6 has lower resolution. S6 carries great screen ever, while G4 boasts of Quantum technology.

Camera
In camera department HTC One M9 is the best one with 20.7 MP camera, however in terms of quality of snaps taken iPhone has better camera. The 16 MP camera on G4 has improved image stabilisation. The G4 camera boasts of better settings. It also features a manual mode and the ability to save RAW format images. However, Galaxy S6 snaps provide sharp images with rich color accuracy. It has some great camera features like faster shutter and better autofocus. HTC One M9 comes with Split Capture mode that helps you take snaps simultaneously with rear and front cameras. The 8 MP camera on iPhone gives some great color reproduction. In the camera department, it would be the battle between Galaxy S6 and G4.

Hardware and performance
All three Androids have almost similar processors with G4 offering a clock speed of 1.8 GHz. G4 and HTC M9 employ Snapdragon 808 and 810 quadcore processors respectively, while Samsung has used the in house Exynos octacore processor. With respect to storage, the G4 excels with its 32 GB internal memory and the expandable memory slot supporting upto 128 GB. Galaxy S6 and iPhone 6 offer no expandable memory slot. HTC M9 has 32 GB internal memory with support for expandable memory. All the three models except iPhone 6 have 3 GB of RAM while iPhone has just 1 GB. But it does not make any difference in iOS kind of things.

My View
Galaxy S6 has a dominating presence with its all round performance and Hardware. The iPhone like design on Galaxy S6 has made it one of the sought after devices. While LG has its uniqueness in the powerful camera features and the newly launched Quantum display. Lastly the pricing can also be deciding factor. With iPhone 6 with 16 GB memory priced at Rs. 53000 and a 32 GB Galaxy S6 at Rs. 50000, LG may be able to garner some sales figures with its better placement in terms of pricing.

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.

Display
Apple 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.