Fire Brigade is really the perfect balance of all of them. Lots of neat techniques (dat brick texture!) and lots of neat interior detail (dat second floor!), and it looks nice once done.
Grand Emporium is the best looking from the exterior (dat everything!), with some neat little details (dat first floor!). But the interior is lacking compared to others.
Pet Shop is more balanced like Fire Brigade is, but is quite different because it is essentially two smaller buildings squashed together. Still fantastic though.
Town Hall is the most bland of them all, but it is still better than many sets out there. It is just that in the company of other Modulars it is less than. However, once it is complete it towers gloriously over the others and looks great (dat dark orange!). Town Hall also has the best roof build of any of them. (Dat skylight!)
Haunted House is a very different build than the modulars because of the hinge design of it, as opposed to the removable floors of the Modular series. It also is more of a three dimensional display than the Modulars because of the hinge design, so you are more conscious of all the sides of it.
Short version: BUY THEM ALL! But get Fire Brigade first, as it is going to disappear first. Even if the rumor was Grand Emporium, Fire Brigade has been missing from the last few LEGO catalogs.
