Glowstone or Sealantern
Sea lanterns emit a light level of 15 which is the same as glowstone, redstone lamps, beacons and jack o'lanterns
A lava lake
Glowstone Witz Trapdoors
Glowstone it looks cooler
Glowstone is the safest but I think lava is the brightest if you can work it into your build
glow stone is certainly the safest...