Primers
Organised by the problem you have, not by product name. Nobody searches "PVA primer"; they search "how do I paint over a water stain".
Water stains on a ceiling or wall
A stain-blocking primer. Latex primer will not hold back a water stain — the tannins and minerals bleed straight through, often after the topcoat has dried.
Fix the leak first. Priming over an active leak just hides it until it comes back.
Glossy or slick surfaces
A bonding primer. These are formulated to grip surfaces a normal primer slides off: aluminium, vinyl, PVC, ceramic tile and glass block.
Scuff-sand first where you can. Bonding primer plus a de-glossed surface is far more reliable than either alone.
New, unpainted drywall
A PVA drywall primer. Bare paper facing and joint compound absorb at completely different rates, and a PVA primer evens that out so the finish coat does not flash at every seam.
This is the one case where a cheap primer is the right primer — its job is uniform absorption, not adhesion or stain blocking.
Smoke damage, nicotine or persistent odour
Shellac-based primer. It is the hardest and fastest-drying stain and odour block available and the correct choice for severe cases where a water-based stain blocker will not hold.
Ventilate aggressively — shellac primer is alcohol-based and the fumes are strong.
Tannin bleed from cedar, redwood or knots
A stain-blocking primer, shellac-based for knots specifically. Tannins in cedar and redwood migrate through latex and appear as brown patches weeks later.
Spot-prime knots with shellac even if you use a different primer over the rest of the board.