It's ranked #1 in score for a reason.
It's so basic...so simple...so straight-forward. One might say that it's been done before--and it has; but NEVER has it been done so well as this. It's safe to say that this game has re-invited the zombie shooter. Other than making suggestions on what to put in for a second installment, there's not enough things this game does wrong to make a decent argument for. Well, no, there are a few things to gripe about. :P For instance, headshots mean nothing in this game, which is unrealistic and a downer for headshot addicts. Also, with the exception of the dog and soldier, there is no variety in the zombies. Every zombie behaves just like the other and it only takes a few simple shots to nail it to the floor.
What I'd like to see, if we're graced with a sequel, is an expanded storyline. What if the army chopper DIDN'T come at the 20th day and they had to move? It would've been neat to see them go into the city, maybe hold up in a mall somewhere, acquire some more help, and take down different types of zombies. Also, a bit of personalization with your fellow zombie slayers wouldn't hurt either.
There's nothing more that can be said about this game that hasn't already said by the 1,200+ reviews before this one. This game has made the defensive shooter fun again and it is really a gem of Newgrounds games. This will earn itself a VERY VERY VERY RARE spot in my favorites folder. Congratulations.