Which water purifier to buy?

Water is one of the most important thing in our life.

If drinking  water is not pure, you and your family's health may get affected, so you should be using the best of best water purifier which will cost you just 15000-20000 , off course that amount is not as valuable as your family health.
That's what you watch/hear in commercials of allwater purifier manufacturing companies.
But before you listen to any of them or get impressed by a commercial of any water purifier,
Know what you need, which depends on source of your drinking water ( Muncipal/Corporation supplied or Borewell) .
Summary(Final recommendation) :

If your drinking water comes from Muncipal/Coporation : 
Any water purfier that UV purification stage is good enough for you.
If you are drinking Ground water( borewell,well) get your water's TDS level tested and if your water's TDS level is below 500, you can still go for UV filtration based water purifier,  if you water's TDS level is above 500, it's recommended that you go for water purifer that has RO filtration stage.
Detail explanation: 
Water Purification stages in home water purifiers:  
  • UV purification
  • Reverse Osmosis
  • Activated carbon filtering
  • Distillation
  • Ion exchange
  • Electro deionization
Not all of them may be necessary for you, it depends on the source of drinking water.

RO is most important(wrong): Water purifier with RO filtration comes 5-8K costlier than the general one (UV filters) , and most people consider RO to be best without knowing if it is actually needed or not? ( and there are some people like one of my friend who says ...costliest product is the best product, because it must be having more/better qualities than other ;o). 
"Gaurav bhai ki Jay ho" ).
What is TDS?
TDS stands for Total Dissolved Solids.
What is the recommended level of TDS?
If you are drinking a water which already has TDS level between 100 to 500, that good enough,
If you are drinking a water below TDS level 100, you missing many minerals and salt that you body is suppose to get, and you are advised to increase salt intake to fulfill your body's need.
What is the TDS Level of my water? 
Municipal corporation water: If you are drinking the water provided by your municipal/corporation , your water is most probably suppose to have TDS level below 500, and you do not need and RO purifier, and if you are still using RO purifier, purified water's TDS level will be below 50, which is not good.
If Municipal/Corporation is providing you water it means, it has to be less than 500 TDS,
As per Mr. Sunny(associated with research water purification industry )  highest TDS level of municipal/corporation  supplied water is in Kolkata which is 450, most of the city's are in the range of 150-350.

Ground water( bore well,well ) TDS level : In general it is suppose to have high TDS  level ( 500-1000) and may need a RO purfication,  but there is no thumb rule that ground water's TDS is always high , so it's better to get tested. some places it's ground water TDS can be as low as 300-500, like some areas in my native Indore(M.P.)

Where to get TDS level tested in Bangalore?
You can get in touch with Public Health Department of Bangalore Municipal Corporation or analytical laboratories like Public Health Laboratory (BWSSB) 22210248, Mineral Engineering Services - 25432968, 9448024304, Cosmic Industrial Laboratories Ltd - 28520541, 1522, 1523, Essen Labs - 23341230/1567, Crystal Aqua Purity Systems (CAPS)- 23362882, 28601762, 28603450, Bangalore Test House - 23356415/23388895/23502684, Viswas Environmental Laboratory - 080-25262987,25277870. These laboratories conduct variety of tests on taken water sample for impurities, TDS, turbidity, minerals (calcium, magnacium, Ph value, chlorides, Nitrates, Arsenic, pesticides, bacteria etc. It costs around Rs 1,000 and onwards. Electrodes used in on-spot testing may be meant for pH levels, but doing through testing will give reading on variety of components and their permissible or ideal limit in water

but after you get it tested please post the result here as comment.
Source: here

How long purified water would stay good if stored?
Answer to this question does't come from any source, this is my own and personnel view.
I think we should not keep purified water more than 24 hours,

I currently use a UV + Carbon purifier and once I have observed, larva in the purified water after 4-5 days..

Perhaps the reason for this was that I did not get the filters changes from long time 8-12 months or the water consumption was less during these days and stagnant water can always get infected. 

requesting reader of this post also to conduct similar experiment, do not cover the glass/mug/bucket completely, let there be some place for ventilation, 
and also do not keep it completely open also,  else life in water can come from outside too ;o)

Best price of water purifier?
I personally prefer referring to Compare India before buying any electronics item, for two reasons,
one is I can compare the items based on their feature and second is i get to know "almost correct best price"
Being an Indian I encourage you to still bargain on the best price mentioned on compare india :o) .