choise default option doesn't work

The default is only shown if it is one of the listed values, which in this case are returned by a tcl block. If you log the list returned from get_items, there will be some difference, most likely that the profile is known as /Common/tcp-wan-optimized.

Of course, i should have thought of that. How can I log those ? thanks

The standard output from iApp implementation code logs to /var/tmp/scriptd.out. You could do something like this:

choice tcp_wan_name default "tcp-wan-optimized" display "xlarge" tcl { 
    set rval [tmsh::run_proc f5.app_utils:get_items /ltm profile tcp]
    puts "tcp profile list: $rval"
    return $rval
}