I use the realclearpolitics.com 4 way average, only I alter it by excludng the widest margin outlier and the narrowest margin outlier from the average.
Currently that means averagng 11 national polls.
http://www.realclearpolitics.com/epolls/2016/president/us/general_election_trump_vs_clinton_vs_johnson_vs_stein-5952.html
That means I excluded Monmouth College’s Clinton +12 and Investor Business Daily’s Trump +1
Doing that for brexit would have made it +6% off actual results, 2014 was 4 Pts off actual results and 2012 was a bit over 3 Pts off. Polls were off even more in 96 (not winner though) and 80. IBD has constantly been accurate on presidential elections and I believe 538 rated them most accurate in each of the last three.