I don't think there is a 'best' wash-in water repellent, but that doesn't mean there isn't. The Gore-Tex® company has laboratories with rows of washing machines and dryers (looking like a high tech Laundromat...) and all kinds of test apparatuses. They can probably determine which of the wash-in products was the best.... and similarly each of the manufacturers of these products must also have their own laboratories. All these labs are probably able to provide comparisons that would be useful to determining a 'best'. We don't have that capability. I use the Nikwax products, more out of habit than anything else. I've used these products for about the last ten years. Before that I used spray-on Heavy Duty Scotchguard for a water repellent treatment, and Tide powder for laundering.
My friend Henry S. Winkour recently asked about re-waterproofing midway through a season, and I told him this is a 'Mr. Subjective' question. I re-waterproof every time I wash my gear, but I do not wash my gear too often. Washing gear is hard on it in terms of the ultimate life of the fabric, stitching and taping (washing a vehicle too frequently is the same...). This does not mean it doesn't feel good to have a clean bike or a clean riding suit. It does. But I have not washed my current darien pants in over a year. They are filthy and could sure use a wash. When I do it next, I'll use Nikwax for water repellent to boost the water repellent that was in the fibers of the Cordura when the fabric was manufactured. How often you wash and use repellents is subjective. This year I did not wash my bike once. I rode about 10,000 miles, which is average. As winter approached I told myself I really needed to clean it up before putting it away. Fall came. The leaves fell. The ground froze. Turkey was eaten. And now it is 20 degrees and there's snow all around my home and the bike is still unwashed. I did all the oil changes and tire changes and regular maintenance it needed during the summer, and at all the appropriate times, but I never got around to washing it. Now I go down to the garage and look and it's dark and thickly covered with that ugly gray film of road filth that motor vehicles get if you don't wash them for a long time. Plus bugs and brake dust and a crust of oil in a few places around gaskets. It's a dirty, grubby, worn bike - and now I'll have to look at it this way until spring, probably.– Mr. Subjective (November, 2006)