Kipchoge is I suppose an easy pick. Kamwo had a phenomenal season, maybe even relatively better than Kipchoge's. I still am not sure how much to value the Breaking2 time, especially as Tadese and Desisa did not show signs of any improvement in their fall marathons. So did Kipchoge really run an out-of-this world time, or was it roughly equivalent to a 2:03/WR-neighborhood time? I think the latter. Geoffrey Kirui was relatively dominant. WVN could have been a solid runner up as well. Seems that as a result of the 'wacky' world champs (especially with the running events) there are less significant contenders, who would have otherwise capped their seasons with a world title.
At first I thought Jepkosgei was a strange choice. However, despite not world title she had one of the best overall seasons. The only other contender would be Semenya. I am of the camp against her competing against women, so I would not have picked her over any woman.
Of course their were some field athletes who I would say had as good or better seasons then Kipchoge, Jepkosgei, or any runner.