Sunstone would give off light equal to daylight (not sure where that lies on that light scale and what other light sources emit) but also acts like sunlight to harm Zombies and Skeletons, also pacifies spiders
As for recipe, I am thinking a base of several glowstones, perhaps make a new block that combines 9 glowstones into a single block and then using several of those new Glowstone blocks.
Could also be redstone compatable (such as a lamp) where it can 'surprise' mobs for an effective defense strategy
The sun is 15, same as glowstone. The sun also does not pacify spiders, the light in which the spider is located does, meaning that spiders in a room with nothing but torches or glowstone will be passive.
As for the recipe, it sounds too expensive for a block with no additional effect other than burning two mobs. If I need to fry them, I'll use a flint and steel.