This may sound really wuss of me, but although I enjoy (if that's the right word) the likes of Ditchling or White Down (never really ridden in mountains, so you'll have to make do with that) what I really enjoy is very slight inclines say, like the one from New Cross to the South Circular through Brockley and Crofton Park, where you can stamp on the pedals and feel the legs burning whilst you fly along at a fair old clip - the lattermost of which is never going to happen on Ditchling, White Down or most of the climbs named above. Doubtless, some would not even regard this as climbing, thobut.