I don't think I could narrow it down to one single product, but there are some I grab more often than not.
P&S Express - I bought this with no expectations and was instantly impressed. I know some hate the scent, but I love it. I also enjoy how Express can be dialed up or down depending on the cleaning ability required.
Gyeon Wet Coat/Carpro Hydr02 - mainly for wheels, but it's a quick and easy way to add something to the paint if required.
P&S Bead Maker - I don't care about the "beading" or the lack of longevity, I'm only using this for the gloss and slickness, for which it's hard to beat. Smells nice too.
ONR - I don't really use this for rinse-less washing, but more so in the quick detailer format for cleaning door jambs, engine bay wipe overs, interior cleaning, work bench wipe downs, I even use it within the home as well.