Not only the outside of Hallgrímskirkja is ... imposing. The organ is also crazy. It's the craziest organ I've ever seen, although admittedly I haven't seen many organs in my life.
(As it says in the temple dome <for those who can read Arabic>), the Greatest Name is "Ya Baha'u'l-Abha," which means "O Thou Glory of Glories" or "O Thou Glory of the All-glorious."