Here are mine:

• -Wextra and -Wall: essential.
• -Wfloat-equal: useful because usually testing floating-point numbers for equality is bad.
• -Wundef: warn if an uninitialized identifier is evaluated in an #if directive.
• -Wshadow: warn whenever a local variable shadows another local variable, parameter or global variable or whenever a built-in function is shadowed.
• -Wpointer-arith: warn if anything depends upon the size of a function or of void.
• -Wcast-align: warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries.
• -Wstrict-prototypes: warn if a function is declared or defined without specifying the argument types.
• -Wstrict-overflow=5: warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. (The value 5 may be too strict, see the manual page.)
• -Wwrite-strings: give string constants the type const char[length] so that copying the address of one into a non-const char * pointer will get a warning.
• -Waggregate-return: warn if any functions that return structures or unions are defined or called.
• -Wcast-qual: warn whenever a pointer is cast to remove a type qualifier from the target type^*.
• -Wswitch-default: warn whenever a switch statement does not have a default case^*.
• -Wswitch-enum: warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration^*.
• -Wconversion: warn for implicit conversions that may alter a value^*.
• -Wunreachable-code: warn if the compiler detects that code will never be executed^*.

Those marked ^* sometimes give too many spurious warnings, so I use them on as-needed basis.