Asylum is a better whole package on account of being completely original and not having any extraneous parts. It just does what it sets out to do and doesn't really stray from that, doesn't overstay it's welcome, just good all around.

City is more bloated (as many sequels are) but if you are purely interested in the gameplay systems, it's the best one. The riddler challenge maps are an entire game all on their own and add a pretty good amount of depth to otherwise simple systems.