Great thread idea!
Agree with a lot of Owen's comments: I get nervous enough about heights anyway, but Senate House library seriously gives me the willies. Narrow corridors that seem to go nowhere, lightless staircases that float between floors, surprise ankle height views down the side of the building. The stuff of Escher's nightmares. Bonus points for doubling as a New York hotel in Jeeves and Wooster.
Adore the South Bank Centre; I'm very uncertain about how the makeover is shaping up though. Any thoughts?
Some favourites of my own:
Canary Wharf tube. Breathtaking.
Alexandra Palace: a half-derelict, hubristic, beautiful, useless extravagance. Catch it before it's sold and turned into a casino.
St Giles Without Cripplegate Church (in the middle of the Barbican). A pretty unremarkable church (inside at least), but I love its location, the way it sits amongst the old London Wall, the Barbican and Richard Rogers' 88 Wood Street.
(that's actually the Simmonds and Simmonds building in the background, here's the Rogers one:
Loads more besides: I think the legacy of Victorian, Edwardian and Georgian terraces is one of London's most important architectural features - these are outstanding buildings, all anonymous, which do their jobs superbly.