For commuter use, fashion is irrelevant! The best helmet is either white or a highly-visible color (reflective materials are even better), has a round, hard shell (number of vents is not all that important since you will sweat buckets anyway), a secure fastening system, and FITS YOUR HEAD!

For most of us, A BMX/skate style helmet will work no matter the riding position even if it looks "dorky" to some people. Perhaps the best type is a BMX full face helmet, but if one is riding in a really low position/uses drop bars one cannot wear one of these (the visor and/or chin guard will impede vision by cropping out a large part of the critical visual field). In the latter case, the fact a helmet is full-face does NOT create problems with hearing what's going on around oneself and actually can help (by cutting down/out noise artifacts such as wind squeal). Both these types of helmets work better for keeping the weather off, too (a concern if one rides year-round in almost any weather).

If one crashes? A smooth, round surface helmet will slide along the pavement, minimizing possible additional injury. A real problem with the "aero" helmets (and visors which are not designed to break away easily) is that the "sharper" edges WILL tend to "catch" on the pavement/ground and therefore raise the risk of injury simply because the helmet is there.

Not a small problem: What good would it be to protect the head against impact if one does fall but to end up paralyzed anyway because the helmet's edges "stuck" on the pavement and one's neck got broken?