Depends on what you're looking for.
A baby dragon should be on a solid substrate because they like to lick everything (and so are more likely to eat their substrate) and they also have a more delicate digestive system which makes them more prone to impaction when they eat things they shouldn't.
The most common solid substrates are paper towels (inexpensive, just throw them out when they get dirty, dont' look the greatest), nonadhesive shelf liner (the solid kind, no holes - wipes clean easily, can be cut to fit the tank exactly so bugs don't get underneath), and textured ceramic or slate tiles (can provide a natural rock look, help keep your dragon's nails trimmed, easy to scrub clean).
Once your dragon is an adult you can keep him on any of those substrates or, if you're really interested in loose substrates, do your research and see which ones have an acceptable level of risk for you. Sifted silica free children's playsand and coconut fiber based beddings like bed-a-beast or ecoearth are both common. Loose substrates are harder to keep as clean as solid ones, and pose an impaction risk to dragons who eat it, but do offer some pros as well. The two loose substrates to avoid for dragons of any age are crushed walnuts and calcium sand - both can do major damage to the insides of your dragon if eaten.