The highest value items are Jewelry, especially the variants with gems. A simple malachite necklace without gem is just worth 40 but it scales with metal quality up to 2000 for a platinum necklace. Together with the modifier for Quality, prices can go far above 10000 for a single legendary gemmed platinum necklace.
However, since higher value ore is hard to get early on, mass-producing Statuettes is usually easier. A clay statuette is worth 16 and requires just one clay clump and a Kiln to produce. You can just replace all clay tiles in your fortress with dirt and get a nice amount of clay to turn into statuettes. Later on, Bone Statuettes are a nice way to get rid of the bones of all the butchered Yaks and keeps your Artisan busy.
Another alternative are Pet Rocks which can also be mass-produced and require just stone which you will have a huge surplus of anyways. They are just worth 10 a piece but due to the abundance of stone, producing 50 or 100 of them is no problem.