Trijicon or Meprolight. Go with one of those brands, the biggest, and you will be alright. I have heard that most brands use capsules from Trijicon anyway, but I have standard 3 dot trijicon tritiums on my G26 and love them. They are a nicer sight than the stock Glock sights, well regulated, easy to see in daylight, and absolutely glow in low light.
Expect to spend ~$100 dollars to have them installed. It is worth having someone with the right tools do it rather than risking damage to the sights doing it yourself.