Mine is certainly Amazon - not many aspects of the internet have so much improved my quality of life.
This began even back when Amazon was about books. In 1998 I visited the USA, and bought two stuffed US Mail Bag's worth of books to be shipped back to England - books about RW Emerson and the New England Transcendentalists that were essentially unobtainable in the UK (except at ridiculous prices - or by travelling to Hay on Wye Book Town).
Just a year later I realised that the situation had changed, and within a couple more years the world was My Oyster so far as second hand book availability was concerned. Of course I will still travel an hour to browse a really good bookshop, as a 'fishing expedition' - but the actual business of getting a particular title to read has been transformed.
However, the main value of Amazon is revealed at this time of year in buying Christmas presents. I have discovered, to my surprise, that I really enjoy buying people presents (so long as I know what they like - which isn't always the case) - but I never previously realized the fact because I really hate shopping.
(Except for secondhand books, of course - but that isn't 'shopping' - it being more akin to a religious ritual.)
So Amazon lets me get presents from a huge range at a reasonable price - without shopping. My pre-Christmas experience has gone from one of utter dread at the prospect of facing massive crowds and queues to get a present that is... not quite right - to a positive pleasure at 'giving'.
No other internet platform or medium has made such a plus-difference to my quality of life.