I was awestruck with Westminster Abbey. There is so much history buried within this structure, and so much more that took place in and around the abbey. I found it amazing to run my hand over worn-down stone that was thousands of years old. Also, having been an English major in college, I found it fascinating to visit Poet's Corner, where many of my literary heroes are either buried or acknowledged in stone.
The lines and large crowds are other aspects of Westminster Abbey that can be pretty unbelievable, but have patience. It is worth the wait.