I live in NYC, and I've just taken to getting everywhere on a bike. I'm riding a real failure of a mountain bike right now, and I just want to know what the best bike I can buy for less than $300 is for riding around this crazy town. Thanks!
Take a look at what hardcore commuters are riding, they are usually heavily personalised older bikes.
When I moved to the Metropolis from Smallsville, I built up a city commuting bike based on an old touring bike. It is tough, fast, carries all my shopping with ease and less attractive to thieves than a nice new shiny model.
You can base the bike on a non-suspension MTB bikes or a hybrid one, but get the nicest frame you can, with threaded eyelets for attatching stuff.
For flat cities 3x8 gearing is overkill. You can use 1x8, but my choice would be a 7speed hub gear (which needs horizontal, not vertical rear axle dropouts).
You need a luggage rack, fenders and lights, and some fast tough slick tyres on strong 36 spoke wheels. If you want to ride in winter, the clearance to fit winter knobbly tyres may be useful.
You can get your personalised commuter bike from the kind of bike shop which sells better used models, or from various community bike action groups. Im sure NYC has several to chose from.