This is like asking what the "Best" gun cleaning solvent is; everyone has their favorite potion but nobody really can say what the best is. The best we can do is give examples of what has worked for each of us!
Still, it is fun to hear everyone's favorite concoction and their cleaning rituals so here I go....
I do a good rinse with either BC Life or a product called Sport Wash (Cabela's has it) that cleans gently and rinses completely off. I've heard of others using baby shampoo, Woolite, etc. I finish with an inside rinsing with 303 Aerospace Protectant and then a good airing out/drying. Lastly, I spray 303 Fabric Guard on the fabric parts and the Aerospace Protectant on the plastic parts.
The 303 products really reduce UV-caused fade and deterioration. I started using them when I bought my Baja speedboat in 1995. The boat cover still looks like new.
DS