The first and least thick string on basses with four or five strings, and the second one on basses with six, unless the bass has a different tuning.
The strings are, from down to up:
1st - G string
2nd - D string
3rd - A string
4th - E string
5th (when there is a 5th string) - B string
The other day I broke my G string while slapping.
I have to buy a new G string, because mine is rusty.