It's so tough to say who's number #1 in the world these days. Most runners seem to stick to either a flat paced course, like London or Berlin, or more difficult, hilly, and unpaced courses like New York or Boston. Also, runners seem to be affected to different degrees by weather, especially temperature. Because when it comes to warm weather, Stephen Kiprotich seems to pretty much rule, with Abel Kirui a pretty close second. Although granted, Wilson Kipsang did run pretty well in the warm London Olympic marathon, and if not for a poorly judged surge, and a missed water station towards the end of that surge (he returned several metres to pick up his bottle, allowing Kiprotich and Kirui to make up several of the seconds he'd worked so hard for), Kipsang may well have won that race too.
However, Wilson Kipsang is certainly the #1 marathoner on a flat, paced course under cool conditions. One of the many things that frustrated me about New York being cancelled in 2013 was that he was scheduled to run there, quite bravely in my opinion, because other than winning the NYC half, he has stuck mainly to less challenging (terrain-wise) courses. So I would agree with you, in order to really establish himself as the undisputed #1 he probably has to run NY and win, hopefully this year, as he seems to be at the peak of his powers right now. Unfortunately I suspect he will make another world record attempt at Berlin this fall instead.
But one can dream. It would be especially emphatic if he could run NY this fall and beat Geoffrey Mutai. Because in addition to the two guys you mention, I think GM has some claim to be the best in the game. He has won on all kinds of courses: Boston (unpaced), NY (twice, unpaced) and Berlin (paced). He hasn't run particularly well in London, although his placing this year was a bit misleading. Although he finished in 6th, almost 4 mins behind WK, he stayed with WK and Biwott for the penultimate surge, but eventually the pace got to him around mile 23, and he dropped out of the leading trio and lost nearly 4 mins over the last three miles, in the process getting passed by several in the trailing group, including Tsegaye Kebede, who I like, but too often seems to run to place rather than to win. Ironically this is what happened to second-placer Biwott when Mutai put in his last surge to win NYC last fall. When things fall apart for these top guys, they really fall apart.
Anyhow, to be the undisputed #1 marathoner it seems to me you have to win (or at least run very well) in all kinds of races in all kinds of conditions. I loved Haille G, but he was basically just a track athelete, sticking mainly to flat and unchallenging courses like Berlin and Dubai. His one shot at NYC, as I remember, lasted about as far as the Queensboro Bridge. I think Wilson Kipsang has the tools to be the undisputed #1, as he looked amazing in the hills of Central Park when he won the NYC half in the spring of 2013. So I hope he goes for real glory and runs the full marathon in the Big Apple this fall. Because with the number of fast young guys hitting the roads these days, not to mention Kenenisa, another world record in Berlin is not likely to endure for very long.