Not artistic design. Not quality of production. Not durability. Not even security features. It's how many countries they let you into without a visa:
"World's best and worst passports revealed" on cnn.com
To eliminate a bit of suspense: Germany is #1 with 177 countries, Sweden #2 with 176. The U.S. is in what they call 4th place, though it's really in 8th because the compilers of the list count a five-way tie for 3rd (Finland, France, Italy, Spain, UK) as just one place ahead of it. Afghanistan brings up the rear with 104.