Sights are a pretty important part of your rifle, without good sights, it’s useless. Iron sights or optics is where most discussions start on this topic.
All shooters should understand and be able to use standard iron sights. Every serious use rifle should have a set of iron sights, even if only as a backup option. There are several advantages to iron sights. Rifles usually come with them from the factory- so they are not an extra cost, they are robust and durable and they don’t require a battery to work. There are several disadvantages to iron sight as well. They can be difficult to use in low light conditions, eye issues can make them difficult to use for some people, they don’t help much in target identification.
Red dot sights are very popular on defensive rifles. Their primary advantages are ease of use- put the dot where you want the rounds to go and shoot making them very fast on target. They are easy to use at night offering a certain aiming point and can even be used with night vision devices. The better models have been proven to be very reliable. Disadvantages include cost- a medium quality- suitable for defense model will typically run $200 and better units as high as $800. A need for batteries to power them can be an issue, though many have extremely long battery life- 50,000 hours in some cases. They provide no more target discrimination help than iron sights.
Magnified optics- scopes essentially, usually 1-4 or 1-6 power, are the most popular current option for defensive rifles. Scopes have several advantages- many have lighted reticles allowing use in low light. They often have bullet drop compensators built in for shooting at longer distances. Their magnification allows for better target identification and discrimination at distance. They are easier to use than iron sights for people with eye issues. The primary disadvantages of magnified optics are cost and some loss of speed at close ranges though low magnification models alleviate some of this.