Both weeks as #1 and Year End #1 are about being the most regular player, not necessarily "being the best".
Why weeks as #1 are about regularity, not necessarily "being the best in the world": The first half of 2017 Federer and Nadal were the best players in the world, despite the fact that Murray was listed #1.
Why Year End #1 is about regularity, not necessarily "being the best in the world": The most regular player of the year is Year End #1. In 2016 Murray was YE #1 with only 1 Slam while Djokovic was YE #2 with 2 Slams. I think the best player of the year is the one who wins most Slams, while the most regular is the one who achieves more ATP points.
By the way, why are you dismissing Year End #1? Djokovic assersts that Grand Slams and Year End #1 are the two biggest achievements in tennis:
"In my personal opinion it’s one of the two biggest achievements you can have as a professional competitor and tennis player – winning a Grand Slam and being No. 1 in the world at the end of the season" -Djokovic
Who will end the season as top dog?
metro.co.uk