There is a very prominent Mara D’asra and Posek in Flatbush who travels to Eretz Yisroel every few months to present Shailos to Maran Elyashiv Shlita. Approximately 3 years ago he asked Maran Shlita whom he considers to be the biggest Poskim in America. He answered in no particular order, Rav Dovid Feinstein, Shlita, Rav Shmuel Kamenetzky, Shlita and Rav Yisroel Belsky, Shlita. If you consider Canada, there is Rav Shlomo Miller, Shlita.
