I never buy the ready mixed ones, you are paying for water, which is something you can add at home. It usually works out cheaper to buy the concentrated ones.
The good windscreen washer fluids have bug and grease removers as well as frost/ice protection which are very handy.
Car windscreen washers I've used and recommend
Car Plan Demon Freeze & Screen wash are good
Holts concentrated screen wash smells a bit but works superbly at removing summer bugs.
DON'T USE RAINX - the coating it forms makes the screen more likely to mist up and is in my opinion quite dangerous! If you don't use your wipers and are driving in heavy rain the RAINX products work really well, but on those damp mornings where you have a fine mist on the screen and use your wipers is just became a smeary mess NB: (I've not used this for 7 years now, so they may have improved their formula)
I had a mate that used washing up liquid and apart from the lethal foaming bubbles it made, it also stripped some of his paintwork -more specifically it removed the wax protection and he ended up with some rust spots.
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