Here are a few general guidelines:
If you only want to carry one floatant: Fly Dip. It pretreats flies so they will float higher for longer and will also revive soggy flies.
If you’re up for carrying two: it's best to choose one of Aquel/Lochsa/Royal Gel and then add Top Ride. The gels should be applied prior to fishing (think of this as putting a rain jacket on before a storm) and Top Ride will revive soggy flies after they start to sink. The difference between the gels? Aquel is the baseline against which all other gels are measured. Royal Gel is a little thicker and includes a subtle sparkle, and Lochsa has a completely unique viscosity that allows it to pair well with all flies, including CDC.
You’re fishing small, delicate bugs: Blue Ribbon or Loon Dust. These fine powders will both get the job done, but you need to choose your application method. Loon Dust is delivered using an applicator brush which is awesome for applying the dust to only part of the fly and working it into hackle. Blue Ribbon is a shake bottle that will quickly coat flies. Either of these will work as a pretreatment, or as a means of reviving soggy flies.
You don’t want to change the appearance of your fly: Easy Dry and/or Fly Spritz II. Other powders will subtly and temporarily change a fly’s appearance (this also improves buoyancy), and sometimes gels can mat materials or make a fly slightly darker. Easy Dry simply returns a fly to its dry, fresh state. Fly Spritz II quickly delivers a waterproofing treatment (via spritz bottle) that doesn’t alter a fly’s appearance.