This is very hard because I'm having difficulty seperating quality of the film from the effectiveness as a reboot. I liked Shin Godzilla more, but I'm at a loss for how it'll be continued from. Meanwhile, I think Godzilla '14 was an effectively sleek reboot to reintroduce the concept of Godzilla and Godzilla movies (with a world of giant monsters that'll come to light and oppose Godzilla) to the public - add the fact that they're using my preferred Godzilla ('60s take) as the template suits me just fine!
... but if I was Japanese I'd undoubtably say Shin Godzilla, because it takes the uniquely Japanese franchise and reclaims it as being very, very Japanese.