the NY 3000 is supposed to be the best U-lock you can buy. In general U-locks can be broken using leverage - you stick a piece of pipe or something similar in the shackle and twist. However the 3000 is pretty resistant to this style of attack. Plus, if you get a model that is only large enough to fit around the meter and your bike without any free space, you make it difficult for a wouldbe thief to stick a lever in there to begin with.

Personally I like the NY Chain because it resists just about any manual attack and takes several minutes to cut through with power tools. Plus I find it to be more versatile. While I can't lock up to parking meters I can lock it up to just about any pole that a bike can't feasibly be lifted over.