A Rectilinear street grid is usually the most efficient layout for a city.
However, my observations of cities haven't convinced me that the grid is always the best form. If you have lots of mixed use and high density, a grid makes destinations easier to find, and its porosity makes the perfect pressure relief valve for those peak traffic times.
Paris, Rome, and Sydney don't have grids.
They are interesting and walkable. Historically, they were a bit hard to navigate for the newcomer. GPS has fixed that, even for pedestrians. Their streets grew organically around the topography and without much planning. A grid for them would increase utility at the expense of charm.
There's an old neighborhood in Denver called Bonnie Brae that was planned in about 1910 and built out over the next three decades. It has curvilinear streets, a diagonal collector road, but no cul-de-sacs. It was designed this way because the private developer thought it would be more interesting, charming, and exclusive than the surrounding grid neighborhoods. He was right, even though many backyards were reduced to skimpy triangles.
In the 50's & 60's this charm was noticed by the suburban developers, and they replicated the style in their new automobile suburbs, added gates, cul-de-sacs, and high speed perimeter thoroughfares. The charm and walkabilty were utterly destroyed by the car-centric design.
So the grid makes city planning and development easier and more predictable, but folks will eventually get a little bored with it, and it doesn't make a better city by itself.
PS. Don't ever make Denver's mistake of putting in one-way streets to expedite traffic. Most of them have been put back to the way they originally were, two way. Also never give in to the folks who live close to the thoroughfares and want traffic on their streets to be limited some way. The extra traffic they suffer with a couple hours per day causes greater benefit of reduced congestion for the city as a whole.
