What is actually best depends on your lifestyle, sexuality, and whether you are looking for solo or couples play. Masturbatorsare generally designed for solo play, although some can be used in ways that increase stamina and make for longer and better erections - something a partner is likely to appreciate.
Masturbators come in a variety of designs - most are designed to simulate a vagina, anus, or mouth, with varying degrees of realism. Others tend for a more minimal design. Masturbators that resemble an anus are primarily intended for gay men. The wide variety means that whatever your preferences, you can find something fitting. Some masturbators vibrate for extra enjoyment.
Cock rings, on the other hand, are designed for couples play - some of them come with clitoral stimulators attached to increase the pleasure of a female partner. The basic purpose, though, is to extend how long you can stay hard - and thus make sure that you don't lose performance before your partner gets what she (or he) is looking for. Cock rings, basically, mean you can play longer and harder and both of you get a more intense experience. They can also be part of treating premature ejaculation.
Cock rings come in all sorts of colors, in different sizes - because we aren't all the same size - and with a different external design. Some of them vibrate or have vibrating bullets that can be attached. You can get adjustable and glow in the dark. Go for rubber for flexibility or metal for maximum pressure.
There's a reason the cock ring is the most popular male sex toy - you can find one to suit regardless of the size of your dong and regardless of the sex of your partner. In fact, the choice can be almost overwhelming - you might want to spend some time reading reviews and taking into account what kind of play the reviewer was doing.
There are all kinds of other options to increase your pleasure - penis extenders to help remind that size does matter, dildos especially designed for gay men, penis pumps, etc. But the two above are the ones most men go for - and with good reason. For solo or couples play, though, you can always check out the full catalog and find out all of the options for you.