St Paul's 3 times already in my 6 years here.
The best view of London in from the top of the dome. Although climbing the 528 steps is not for the fainthearted but don't despair, there are several places to stops, such as the Whispering Gallery and the Stone Gallery before reaching the Golden Gallery's amazing views.
They have an American Memorial Chapel as a way of saying thanks for our help during the war. Winston Churchill ensured that during the war, that if everything else was bombed, he wanted St Paul's to be protected so he had people stationed there to put out any fires that began. It escaped any major bomb damage and was hit by 28 bombs.
It is also the place where Diana and Charles married in 1981.
The crypt is very interesting as well, it hosts the body of Admiral Lord Nelson (the guy on top of the statue in Trafalgar Square).
I find the architecture breathtaking and will continue to go again and again. The current cathedral was built between 1675 and 1710 and is the fourth to be on this place the first was built in 604.
You can get in free if you attend a service and the church does shut for the services. It also closes early so do check the times before you go there. I hope you enjoy it as much as I do.