You have had many good answers. One I would like to add for the consideration of everyone is poor compression caused by sticky valves.
The springs have to return the valves to closed. Sometimes on some engines, if the compression is not good, and you are going to consider it ruined, there is no harm in opening up the part of the engine that houses the tappets and the springs. I have worked for a few engines for friends that had the following problem:
Oil sludge, gunk, and perhaps a bit of carbon accumulated on the lubricated components of the valves, and was seizing them when cold. I clean, scrape, soak, and lube the area, and it often will start right up.
All I am saying is don't automatically assume the problem is rings, or burnt valves if there is a compression problem. Sometimes it can be a valve that is not moving properly.