When I need to leave my bike (even only for a few minutes) I use the following:
Abus Granit XPlus 540 D-Lock securing the frame and the back wheel as well as the
Abus Granit 53 D-Lock Combo Pack securing the frame and the front wheel with the cable through the rear wheel, frame and seat to the bike rack etc.
Obviously, I only carry these locks when I know I am intending to leave my bike for any length of time at all. Mostly, I don't carry locks because I never let the bike out of my sight on a ride.
However, if I use the bike instead of the car for a trip to the shops or to meet for a coffee somewhere, then the locks come with me.
I know all these locks can be successfully attacked in a matter of minutes, but not without power tools in the case of the D locks. The cable came free and just adds a slight extra deterrent, stopping an opportunist nicking my bike seat which happened to me once - that was a bummer!