Good thread Jactac
I'm finding it really hard to answer this question, there are so many products that I will probably always use but to narrow it down to one, is nigh on impossible.
For me, it would be between Valet Pro Advanced Neutral Snow Foam (but used as a pre wash in a chemical sprayer rather than a snow foam) because it's cleaning abilities are second to none. THE best Pre wash I have ever used. It leaves the car 90% clean without even washing it. It also means that you've removed the worst of the grime and grit therefore reducing the risk of scratching your car when you come to physically wash it. You can dilute this down to 1:30 or use it neat. Even at 1:20, which is how I use it, it's cleaning powers are very impressive AND it won't remove your wax or sealant. It can also be used as a wheel cleaner too so it's 3 products in 1.
The second product would be Gtechniq G1 rain repellant. It lasts up to 2 years or 20k miles. You apply it and then forget about it then when you are driving in the rain, the water just sheets off the windscreen. You don't realise how good this stuff is until it starts to fail then the water doesn't clear and you have to use your wipers more, which I hate.