+1 for copper.
Soft copper refrigerator coil is incredibly easy to bend around a corny keg or equivalent using a cheap spring-type pipe bending kit. Copper also adds necessary trace nutrients to the beer which aids yeast nutrition. It's also very easy to sanitize/sterilize, since you can just fill the copper coil with water and dump it into your wort kettle for the last 20-30 minutes of the boil. Just be careful to catch the boiling water that spits out the ends!
If you're worried about tarnish on the copper, just put it in a boiling pot of slightly acidic water for 15-30 minutes, then carefully rinse and dry it afterwards.
Finally, if you want to upgrade to a counterflow chiller, it's easy to incorporate your copper tubing into the new project.