The simplest way to approach this is to consider the mean wave energy flux in terms of the power per unit length of wavefront, usually expressed in kW/m. In other words, if I could build a perfect wave energy converter, then that is the amount of power that I could get, on average, per metre of device. That's somewhat simplistic, and it ignores many factors such as storminess, depth, accessibility, and so forth. But, answering that question:
The short answer is "the exposed west coasts of most continents", as these have the greatest fetch, or distance across an ocean for swell waves to develop in prevailing Westerly winds.
There are unsourced maps on the websites of a few wave energy device developers (e.g. Pelamis). I'll not reproduce them here as I have no idea of the copyright situation.
An academic evaluation, based on global wave modelling, is in this conference paper (DOI: 10.1115/OMAE2010-20473). Figure 4 shows their estimate of the annual average power per unit length along all coasts, except where it's very low or where there's ice.
- Western Europe
- South Africa
- South and West coasts of Australia and New Zealand
- West coast of Canada
- West coast of Chile. The southern part of this probably has the highest incoming wave power in the world, as it's the only interruption in the fetch around the Southern Ocean.
Moving away from the coastlines, today I happened to come across this GIS tool which, among other things, has a layer showing the average annual specific wave power in kW/m. This data is based on wind modelling / hindcasting and modelled wind-wave interaction.
The image below shows this layer at a worldwide scale:
To give an idea as to scale, the deep red colour in the Southern ocean is "over 90kW/m".
The layer cites the following two conference papers as its source:
WorldWaves wave energy resource assessments from the deep ocean to
the coast (by Stephen Barstow, Gunnar Mørk, Lasse Lønseth and Jan
Petter Mathisen). Proceedings of the 8th European Wave and Tidal
Energy Conference, Uppsala, Sweden, 2009, 149-159.
Assessing the global wave energy potential (by Gunnar Mørk and
Stephen Barstow (Fugro OCEANOR), Alina Kabuth and Teresa Pontes (IST,
Portugal)). Proc. of OMAE2010, 29th International Conference on
Ocean, Offshore Mechanics and Arctic Engineering June 6-11, 2010,
Shanghai, China; OMAE2010 – 20473.