What is the strongest U-lock?

I wanted to start commuting by bicycle to my new job, but a theft of a employees bike is deterring me from trying. His bike was locked up with a U-lock to a thick stair railing out front. During brunch service, a guy walked up with an angle grinder and srltarted cutting. This was infront of a few groups of people waiting outside to be seated. In something like 10-15 seconds the lock was cut and he rode off with the bike. Are there special U-locks made to handle angle grinders or should i just keep driving to work?