How to make wlan to always be the default route? (ip route list)

For changing the routing priority for an interface, you change metrics.

By default, all are 0, which is the highest priority. So, you can do:

allow-hotplug eth1
iface eth1 inet dhcp
    up ifmetric eth1 30

To use ifmetric in Debian, you have got to install it:

sudo apt-get install ifmetric

ifmetric

Set routing metrics for a network interface

ifmetric is a Linux tool for setting the metrics of all IPv4 routes attached to a given network interface at once. This may be used to change the priority of routing IPv4 traffic over the interface. Lower metrics correlate with higher priorities.

The metric 0 means the highest priority route and is the default one. The larger metric value means lower priority routes. The IP address of the active interface with the lowest metric value becomes the originating one. See ifmetric(8).