Broad question. Answer depends on what you're shooting (benchrest paper or snapshot game), what you're used to, what rifle and caliber you're talking about, what lighting you're most likely to shoot under, etc, etc. Most important in sights is sight height (IMHO). If your cheek has to come out of the stock to gain the sights (eg: AR15 for many), your learning curve will be that much flatter.
For quick shots, I like the ghost ring system, particularly those from Ashley Outdoors. I think you'll see as many other opinions here as there are posters. There's no right or wrong answer.
Rich