London for me this year. I run some of the not so pretty bits very often, but they're much more fun with thousands of people screaming at you.
I've done it once but also missed out three times due to injury, so I've got a serious score to settle.
Paris is lacking decent toilet facitlities at the start but other than that it's great. Starts on the Champs Elysees, lots of lovely wide, flat, long boulevards, and you get the Eiffel Tower to look at instead of Billingsgate Market.