The most popular male sex toy is the cock ring - likely because it is extremely versatile, good for couples and solo play, cheap, and easy to transport. They're the simplest sex aid around and work for gay and straight men alike. They increase stamina and some allow for both vagina and clitoris stimulation - very intense for female partners.
However, there are also a bewildering variety of cock rings around. Different materials, vibrating, cock and ball rings, adjustable...how do you know which you should choose?
First of all, size matters. A cock ring that is too small is dangerous (Yes, men have got them stuck before and ended up in the ER), a cock ring that is too large will not do its job of restricting return blood flow to increase erection length. You should measure yourself - or have your partner measure you - with the penis fully erect - either using a tailor's tape measure or a piece of string you can lay out and measure afterwards. The correct size is very slightly smaller than the girth of your penis. For cock and ball rings, you measure behind the balls as close to the body as possible. Double rings go around the cock and balls and constrict both - these are very intense and some men may find they come too quickly.
For your first cock ring, an adjustable cock ring is a good idea, as you can be sure of a fit and you can experiment to find out how much constriction you like. These are also good for gay couples because they can be shared even if you aren't the same size. Adjustable cock rings are generally made of leather. Stretch cock rings are also good. Metal and glass should be reserved for men who are experienced in using cock rings - they are rigid and you cannot break or cut them if something goes wrong.
Vibrating cock rings are designed for heterosexual couples. A bullet or other shaped vibrator on the top of the ring applies vibration to the woman's clitoris - and some to the penis too. They tend not to be as tight a fit. However! They can also work well for gay men - if they put the vibrator on the bottom so it's against their balls. It's a pretty mild vibration.
Teardrop cock rings are designed to cradle the perineum - between the balls and the ass - and are designed to heighten sensation for the wearer.
Experiment! Different cock rings have different effects and they're cheap enough that you can own a small collection if you want.