If we could start our electrical grid from scratch with today's technology, which would be the most efficient choice? AC or DC?
A Bit of History

The suggestions behind this topic go against what a lot of electrical engineers have been taught since their first circuits course - that AC is better for power transmission. After all, in the "war of currents" in the late 1800's, it was Tesla who helped Westinghouse fight for AC, eventually defeating Edison's dreams of a DC empire.

The primary advantage of using AC over DC at this time was efficiency. It became increasingly easy to transform one AC voltage to another, especially when compared to the cost, difficulty, and inefficiency of converting one DC voltage to another at that time. According to Joule's First Law, the amount of power wasted as heat in the transmission lines is proportional to the current squared. Considering the transmission lines have a known, (basically) fixed resistance, then for a transmission of the same amount of power, much more is wasted in a low-voltage, high-current transmission than a high-voltage, low-current transmission. As stated, it was very impractical to convert DC voltages to a high enough level to over come the line loss when compared to the relative ease of transforming AC voltages.

As a side note, many places never fully switched from the original DC transmission systems to AC until the mid 20th century. 

You can read all about the history here.

Enter Modern Electrical Design

This is not to say AC doesn't have it's own problems. The skin effect is one example of AC being less efficient than DC, but it still doesn't compensate for the above mentioned line losses. Another issue is that of corona discharge occurring at high transmission voltage levels. Over long distances, AC power also has stability issues. This IEEE article sites a few different distances, noting that the line reactance can be compensated for in distances up to 600 - 700 miles.

With modern implementations of mercury arc valves, thyristors, and IGBTs, and efficient means of DC voltage conversion, HVDC transmission is not only possible, but overcomes many of the problems faced with HVAC transmission. The overall transmission distance is much greater, and the mentioned AC effects are overcome. In addition, the cost associated with HVDC is less than HVAC, once a distance threshold has been crossed. This cost differential is discussed in detail in this paper which includes a breakdown of power substation cost as well. The cost is also discussed in the link provided by Jake in his answer.

The fact is that the current electrical infrastructure is based around AC power transmission. The vast majority of modern technology requires this type of power for proper operation, and had AC never been used I doubt we would have many of the technilogical "advancements" we known and love. Theoretically, using HVDC alone could prove to be more efficient, but to compensate for the difference in cost, a hybrid HVAC/HVDC system is the best solution, at least in this moment in human development.