A Primer That's Easy to Sand
If your goal is an ultra-smooth topcoat on a bare wood with visible pores, like pine, your best bet is to prime the wood first. A good primer will fill in the pores, and a good sanding will smooth out all the brushstrokes. Trouble is, many primers gum up sandpaper like nobody's business. Here's one that doesn't.
Benjamin Moore's Fresh Start Enamel Underbody Primer is specifically made to sand easily. It's an alkyd (oil-based) paint, so it flows and levels extremely well. It's ideal for doors, cabinets and trim that will be painted with a gloss or semigloss enamel. You'll have to wait overnight before sanding and recoating, however.
Flat Finish But Easy to Clean
Flat paint used to be a bad choice for bathroom and kitchen walls. If you needed to be able to scrub fingerprints, grime or grease off a wall, you had to use a paint with some sheen. That's no longer true. Now there are paints, including Emerald Interior Acrylic Latex and Duration Home from Sherwin-Williams, that are both flat and washable.
These two choices also contain antimicrobial agents to inhibit growth of mold and mildew. According to Sherwin-Williams, Emerald offers better washability than Duration Home. Both are available in satin and semigloss as well.