What are the tiers of the ores in Terraria?

As of 1.2, every world gets either one or the other generated:

Standard Ores can be found almost everywhere underground:

• Copper or Tin
• Iron or Lead
• Silver or Tungsten
• Gold or Platinum
• Demonite or Crimtane

Demonite and Crimtane are rarely generated, so that's why The Eater of Worlds and the The Brain of Ctulhu drops them in large quantities.

Hardmode Ores are generated by destroying Demon Altars with a Pwnhammer. You can obtain this hammer by killing the Wall of Flesh, activating Hardmode.

• Cobalt or Palladium
• Mythrill or Orichalcum
• Adamantite or Titanium

And these ores are always generated:

• Meteorites fall after breaking a least one Shadow Orb or Demon Heart.
• Hellstone is found almost anywhere in The Underworld.
• Chlorophyte is found in the Underground Jungle after hardmode is activated.
• Luminite is dropped by the Moon Lord.

Additionally, Hallowed Bars drop from certain hardmode bosses.

Summary: Luminite > Chlorophyte > Hallowed > Titanium > Adamantite > Orichalcum > Mythrill > Palladium > Cobalt > Hellstone > Demonite/Crimtane > Meteorite > Platinum > Gold > Tungsten > Silver > Lead > Iron > Tin > Copper

http://terraria.gamepedia.com/Ores