Cable locks tend to be the easiest to cut. I use 2 Abus locks when out and about, one is a big bulky chain with built in lock, the other is an Abus Granite Xplus D lock which uses a square section "D" so that you need to cut it twice to remove it (round section D's you can cut once and rotate the remains to release.)
They are both Gold rated, the chain one cost me £90 and the Dlock £80.
I also have a £16 Abus cable lock that I use to lock my front wheel in place as an extra deterrant.
(Ofcourse none of that helps if the thieving scum breaks into you house and steals your unlocked bike, as they did with mine early on saturday morning.)