I dont know what is my favourite product but so far but I really do like Auto Glym's Car Galss Polish.
If you want clarity then this is the stuff to buy, makes windows like new again and even has anti misting agent. Can even improve wiper blade functionality!
http://www.autoglym.com/products/products/car-glass-polishCheck the video out on their website. I bought this originally to remove those nasty water spots that you can get when droplets dry on your car. Your standard glass cleaner spray just doesn't have the muscle to remove these!
As it's a polish this has abrasives in it so it will easily remove these and stains etc. Can be used inside and out really easy to use. JUST DONT USE ON YOUR LIGHTS!
Once your have cleaned/polished just seal with a glass sealant, carbon collective seems popular right now. I use Auto Finsse tough coat for all windows except front windscreen. Not sure why, Auto Finesse say it's for safety reasons not to...