My ranking is the reverse; I prefer this film to the Godfathers, but only by a hair. Goodfellas is third. This is definitely Leone's best, though, followed closely by Once Upon a Time in the West.
I can see where you're coming from (both involve crime, intrigue, cops and robbers), but I don't think it's really same genre.
Don't get me wrong, on its own terms The Killer is an amazing film, basically defining the heroic bloodshed action genre, and together with Hard Boiled and A Better Tomorrow among the three must-see John Woo movies.
They don't go about it the same way, but I think they explore a lot of the same themes -- loyalty, morality vs. the law, etc. I think people kind of undersell the Hong Kong triad movies as anything other than mindless action.
I also consider this to be Sergio Leone's best movie and one of the must-watch movies by any movie fan.