What is the best layer to mine for gold in Minecraft?

I'm always mining on the second level on top of bedrock. That is 4-6 blocks on top of bedrock layer.

This area is very rich in everything. Tons of coal, tons of iron and reasonable amounts of gold/diamonds/lapiz/redstone.

mirror of original source

See also http://digital-trauma.de/mc.html and http://minecraftwiki.net/wiki/Altitude

Gold is equally common between 5-20 blocks above bedrock. Lava becomes extremely common at about 10 blocks above bedrock. If you mine at about 12 blocks above bedrock, your odds of finding ore are just as good as they are anywhere else, while your odds of finding lava are far lower.

So save yourself a lot of pain: don't bother going much deeper than about 12 blocks above bedrock.