How to know which smartphone we should buy?

First and foremost, for the various key parameters I have stated below, each individual will have different priority orders. Just go through the following sub-headings and factor in your preferences and usage, and I hope you'll be able to make a more informed decision. This is an extremely comprehensive guide to smartphone buying, but I’ll keep it s non-technical as possible and focus more on the usability aspect of it all. So let's get started.

PRICE

Before buying a new smartphone, the first thing that we are pretty clear about more often than not is the price range. Even though it is good to be clear about this, always be just a bit flexible about the same, as an additional 2K-3K could make a significant difference and that extra money spent could very well be worth it. 
For example - A person having a budget of 25K INR is better off buying Oneplus 3 for 27999 and that extra 2999 is completely worth it.

SIZE

In all my interactions regarding smartphones, more often than not, people do not give much importance to the size of the phone. When I ask, many times I hear things like, "Anything larger than 5 inches", "Anything is fine", "not too small" etc. It's pretty obvious that the person has not thought through about the size.

Why is this important? 
It's because the user experience can become much better or much worse depending on how apt the phone size is. Some prefer one-handed usage, others use earphones/Bluetooth headphones while talking on the phone, some might use it for media consumption so would prefer a bigger screen, and so on. So, your usage and the size of your hand will determine the optimal phone size for you. This parameter may not be a deal breaker, but it should definitely be taken seriously while purchasing a new phone.

how to decide?
The size of the phone is governed by 2 factors, its screen size and the empty area around the screen called bezel. A large bezel could make a phone much bigger than what the screen size suggests. For example, 2 of the best phones right now, Galaxy S7 Edge and iPhone 7+ both have the exact same screen size but size wise, iPhone 7+ is about 12.5% larger than the S7 Edge which is a significant difference. This is mostly due to poor space management by Apple.
I highly recommend every single buyer to try and get hold of the phone (or a similar sized phone) you are planning to buy at least once before you actually buy that model, get a hang of the size, and then decide.

CAMERA

This is the most tricky one as just by reading specifications, it is impossible to judge the camera quality and these days, most of the smartphone options are quite similarly specced. The good thing is that these days, even the phones in the price range of 10K-15K INR have decent camera quality.

Please note,

1. A higher megapixel count doesn't mean a better camera at all. So, just ignore that number. However, this logic is somewhat true as far as selfie cameras are concerned. However, a 5/8 MP selfies camera in a mid/high range phone should be decent enough for most people.
2. You don't need as good a camera on your phone as you think you do. If you carefully think about your camera usage, you'll realise that you don't actually use your camera that often and might want to have a good camera phone just for the heck of it.
3. When we see the photos taken on the phone only, the display quality of the phone makes a huge difference in the perceived quality of the images and could be a wrong indicator of the camera performance.
4. Even though most of the experts and reviewers will suggest otherwise, the difference in the image quality of competing smartphones in the same price range is not too much for an average consumer.
   Redmi 3S Prime/Redmi Note 3 - Not too big a difference.K5 Note/Galaxy J5 - Not too big a difference. Galaxy J7/Moto G4+ - Not too big a difference.Mi5/OnePlus 3/Honor 8 - Not too big a difference.Galaxy S7/iPhone 7/Pixel/G5 - Not too big a difference.

To reach a decision, please understand that what matters more is the usability rather than the capability of the camera. So how to reach a decision? Well, consider following steps -

1. Unless a better camera is your #1 priority, it is better to reach a shortlist based on other factors that you consider important.
2. Among the shortlisted options, if everything else is equal and only the camera has to be the tie-breaker, then just google a comparison between the shortlists and I am sure you'll be able to find a good enough comparison on some website or youtube. Quickly go through that.

BATTERY

Battery performance is something which everyone would straight away say, the longer lasting it is, the better! I completely support this thought process. However, first, you need to understand clearly what your lifestyle would be for the next 2 years of so.

It is also crucial to understand that a larger battery doesn't necessarily mean that the phone will last longer. The phone's software optimisation, processor efficiency, display type and your usage have a much larger impact on how long a phone will last for you.

For eg., Moto X Force (3760 mAh) has a battery which is 25% larger than that of Xiaomi Mi5 (3000mAh) but even then, Mi5 is almost on par, perhaps even better in terms of battery life performance, even though it is much cheaper.

Please keep this in mind -

1. If you have an office job not involving too much travel, a phone that could last 1 day (8 AM to 10 PM) too would be fine for you. A healthy share of phones these days are capable of doing that.
2. If you travel too much or due to some reason, can't charge your phone during daytime at all and long lasting battery is an absolute priority, you might want to consider phones with huge battery sizes (3500+ mAh).

However, I highly recommend a power bank to everyone concerned about their phone's battery life too much.

DISPLAY

The displays in smartphones have evolved significantly in the last 4-5 years and these days, even a 10K INR phone has as many pixels as that on a 60 inch TV !!!

Having a good display is important because -

1. It has a huge impact on your phone's battery life.
2. It can make the entire content that you consume look so much better. Images, web pages, UI, games, you'll notice a huge difference in it which you never noticed so far. Try this out - take any low/mid range phone with LCD screen (Redmi Note 3 types) and compare it side by side with a phone having AMOLED screen (J7/1+3 etc). Open a bright, colourful image of a landscape on both and increase the brightness to full. Notice that huge difference? Insane, isn't it?
3. It can make a big difference when you use your phone outdoors in bright sunlight.
4. Even though sharpness is not a major concern these days as most of the phones' displays are pixel-dense, but there is still a significant difference in a 720p display and a 1080p display.

So How to decide then?

1. An AMOLED display with the same resolution is almost always going to be better than an LCD one (few exceptions - iPhone/HTC 10/LG G5 displays which are also quite good).
2. 720p is good enough till 5-inch screen size and 1080p for beyond. In my opinion, 1440p displays (Galaxy S7, LG G5, Note 5 etc) are an overkill (if you don't plan to do VR stuff). But never go just by the screen resolution. Sometimes, even a 720p screen could look better than a 1080p screen.
3. The single most important aspect of a display is its contrast ratio (colour calibration and brightness too is very important). Do keep this in mind and google about it while comparing 2 phones with an LCD screen.

LOOKS & DURABILITY

These parameters are highly subjective and the only points I can suggest is -

1. If you generally do not mind putting a back cover on the phone, then please do not rate this parameter highly while decision making.
2. Owning a beautiful phone made of glass body is fine but please do consider how careful you are while using a phone. With beauty comes slipperiness, especially glass phones tends to be much more slippery, and hence more risk of dropping (unless of course, you use a cover).
3. A phone with even a Gorilla Glass 5 is not damage-proof, it will crack if it falls face down. The only truly damage proof screen is that of Moto X Force but alas, that phone is not a value for money.

Note - Tempered glass on your screen, ASAP.

OTHERS

There are several other things to consider while making a decision. Some of the most important ones are -

1. The UI of the phone. This is where Low/Mid range Samsung phones sucks big time. They are generally too heavy for the processor. For eg., Moto G4's UI will feel much more fluid to you than that of Galaxy J7 even though they cost almost the same and the processors in both are equally capable. Also, Xiaomi's UI is not everyone's cup of tea, so check it out once before buying a Xiaomi phone (I agree that this is too much micro detailing!!!).
2. A phone which promises future software updates is to be preferred over others, this is what experts say. But I disagree with that to a large extent. Reasons - a) Android Nougat was released in August and even till now, only about 5-7 models are running Nougat. Even the mighty Nexus 6P didn't get Nougat update yet. The delay is so much for the flagship phones, for the mid-range phones, it is just too much to even factor this in.b) Even after these updates, the difference in the user experience is not as significant as we expect (few exceptions do exist).c) Other than for flagships, we can never be sure about which model will receive the update and which won't.
3. Water-proofing, absence of headphone jack, micro-SD support, 4G support, Dual SIM support, Fingerprint sensor etc are other minor factors that you can take into consideration based on your preferences.

That's it from my side. I hope you find this answer useful and it helps you make a better decision