RWE – I meant “large” as in biggest yield. I think you mean “heaviest.” Wikipedia claims the B41 had the highest yield of US weapons:

http://en.wikipedia.org/wiki/List_of_nuclear_weapons

Personally, I’m skeptical of the degree of damage from EMP to civilian infrastructure. For the the electric grid in the US, there are a number of design features that would be protective against EMP. Solar magnetic storms share certain characteristics with EMP and we’ve done some work on that since North America had a couple of bad experiences a couple of decades ago.

Back on thread, just how would we rate the odds of an Israeli first strike against Iran? I’d say it is pretty low. As Kinuachdrach and others have noted, sometimes all you need is an ice pick, properly applied.