Zinsser’s line of stain covering primers is the best in the market. If you have a stain to fight, then they’re the company you want to do business with. This is a synthetic shellac primer. The great advantage to that is the fast dry time of shellac, not allowing the stain an opportunity to soak into the primer. I've used Zinsser’s products for years to block stains, with excellent results. While many people use it for merely spot stain blocking, it is an excellent primer for everything else as well. Excellent adhesion to almost all surfaces ensures long life and it is even capable of blocking odors as well. Cleans up with isopropyl alcohol (rubbing alcohol)..
Kilz produces a wide variety of specialty primers. Like Zinsser, some of their primers are shellac based, providing the same stain hiding capability. This one is a latex primer, which has been created to provide the same advantages as a shellac primer, with the convenience of water cleanup. While I can’t say that it quite matches shellac for difficult stains, it still does an excellent job in most cases. Although latex based, it can be top-coated with either latex or oil-based paints.
Pratt & Lambert Multi-Purpose Waterborne Primer
This primer, from Pratt & Lambert, is specifically formulated so that it can be used both indoors and outdoors. It bonds extremely well to almost any substrate, including such difficult surfaces as masonry, aluminum and PVC pipe. Please note that some of these surfaces may require special surface preparation for optimum sealing. Another difficult surface that it works extremely well for is repainting over existing high-gloss paint. It is a high pigment, stain hiding primer, which eliminates the need for a separate stain blocker. Fast drying, it can be top-coated with both latex and alkyd paints.
For a normal primer, which isn’t specifically intended to be a stain hider, Benjamin Moore’s Fresh Start is an excellent product. This primer is formulated to provide a surface for painting, as if you were working on new construction. That’s great when you’re trying to repaint something, especially if there are problems with the previous paint. It is a low VOC product, which dries quickly. Fresh Start provides excellent adhesion and sealing in an easy to apply product.