Republic Bikes Aristotle
In terms of popularity, Republic Bikes made a name for itself as essentially the Nike ID of fixed-gear and single-speed bikes. This was due in part to their partnership with hipster superstore Urban Outfitters. Their models range in price from $399-$519, so almost all of them could have landed themselves here, but the basic Aristotle has a pretty good bang to buck ratio. You'll end up with a Hi Ten steel frame, flip/flop hub, 30 mm deep rims, riser bars, and brakes for a hair under $400.
Buy It Now at Republic Bike, $399