Coal can be replaced with wood.
Lapiz is useless
Emeralds come from villagers
Iron: I get tools from villagers or mine with TNT.
Diamond: villagers sell diamond tools too.
Gold is easier to get from zombie pigmans
Hrm, emeralds can be received from villagers. And iron golems drop iron, and villagers sell diamonds. Lapis is useless, as the above poster said, as with coal. So either gold or redstone. But both are drops from mobs. (redstone trades from villagers are useless) So if I had to choose, I'd say redstone as well.
"Sometimes, I just wanna give up, say 'I'm done with this mess' and go to bed. But you know what; you can't shrug off your responsibilities. You got to pull yourself up and meet the challenges head on. That's the only way you're gonna get ahead in life."
Iron. its tools are both useful and cheap, I would never make armour out of any other material, because diamonds are too expensive, and gold and leather are a waste of material. Also... buckets, rails, minecarts, and many other things would never be able to be made otherwise.
While the people above are right, redstone is hard to get otherwise, it's more useful in creative anyway.
EDIT: and, I use charcoal more then real coal anyway.