My usual is the 2.5 litre Tesco own-brand concentrated stuff in a 1:1 concentration. It's cheap, does a fair job of keeping grease at bay and shifting most muck, but it really struggles with bugs - usually changing them from an annoying dot to a really annoying white arc. It also stinks!
I've just finished trying Halfords super-concentrated lemon scented stuff in a 1:4 concentration. Clears bugs and road muck well but I'm having to take the fairly liquid to the windscreen every other day just to de-grease it.
Fairly liquid is the daddy. With a spot of water it cleans brilliantly and de-greases perfectly. Only *itsy weeny little problems* are 1) you need paper towel to rub it in and clean it off - not easy on the motorway unless you're in some sort of action movie and 2) you don't ever want to put it in your washer reservoir
