I have over two dozen cock rings, straps and stretchers made in a variety of materials. Leather, neoprene, latex, metal, silicone and even carved bone...
Anything tight is impossibly uncomfortable, and probably unsafe to wear for prolonged periods of time.
For daily, all day wear I use metal cock rings in the 2" to 2 1/8" diameter range. They feel good, the weight and slight pressure makes them noticable to the wearer, and depending on the thickness they can make your package bulge a bit. These are easier to put on than they look, especially if they're sized right. Measure around the base of your cock, behind the balls using a piece of string - mark where the two ends come together, then measure the distance between the marks. Take that measurement and divide by 3.14 (pi). This will give you the approximate diameter of the cock ring you need.
For reference, on the small side a measurement of 5 1/2" needs a cock ring that's 1 3/4" diameter. The average case is is about 6 1/4", fitting a ring that's 2" in diameter. Larger guys in the 6 5/8" range need a 2 1/8" ring. For all day use, you want a ring that fits a bit loose.
I also have a combination cock-ring/thong that's fantastic for all-day wear. Check out Koala Swim Wear, I have one literally called the "Blow Me" suit. This really enhances your gear, and is comfortable enough to wear most of a day.
Here's a metal ring for all-day wear: