Tricky one. The titanium Planet X is by far the nicest but it is much easier to say which I don't like.
Brompton: Brilliant for folding up and sticking under my desk or taking on the train when I have to go to my other office. Just not brilliant for riding and a right pain to carry.
Dave Yates: Fine bike but it has mudguards and so gets ridden in the rain. In consequence I have taken against it a bit, which is a little unfair because it really is a good bike.
Specialized Langster: Absolute blast but it is so rigid that it beats me up after an hour or so.
Cotic Roadrat: Winter commuter, so mentally associated with dark, cold, and wet.
I love them all.