Kryptonite U-lock
Pick one really, but if you want it to actually take more than 5 minutes or power tools to break you'll have to get the NY Fuggitaboutit which is overkill for most everyone unless you, well, live in NY.
I use the older version of this (still post pen-cap issue):
https://www.kryptonitelock.com/Produ...=1000&pid=1205
And augment with a basic cable when I don't want to pull my front wheel off.



I am not locking up anything of great value currently but was unpleasantly surprised at how easily I was able to cut my own lock (yes, I was extremely frustrated but still...). Plus I saved the $45 the campus bike shop wanted to charge me 
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