Brush choice depends on what the brush’s job will be (transferring paint, smoothing paint, etc.), the user’s preferences, and the project budget. The best brush isn’t necessarily the best brush for the project. You wouldn’t use a $40 brush that requires meticulous cleaning to paint the bilge or apply bottom paint, just as you wouldn’t use a throw-away chip brush to lay a final coat of varnish on the toerail—at least we wouldn’t.
For varnishing: You need a brush that is dense, has a chisel tip, and absolutely will not shed bristles. This could be a good badger-hair brush or even a foam brush. Both Casey and Rebecca Whitman (professional wood finisher and author of “Brightwork, The Art of Finishing Wood”) extol the good performance, easy cleanup, and environmental friendlier-ness (no solvents required for cleanup) of the Jen Poly-foam brushes.
For painting: Foam brushes won’t fly with polyurethane paints, plus you want the brush to hold plenty of paint, so a bristle brush is better. Disposable, throwaway China chip brushes are adequate for small jobs or those where perfection isn’t a priority. If you’re rolling-and-tipping a hull and will realistically follow a brush cleaning regimen, then an oval-shaped badger- or China-bristle brush with a chiseled, fan-shaped tip is best.
For more advice about painting your boat, check out the Painting Your Boat From Bottom to Top complete series by Practical Sailor.