I will not pay stupid money for retired sets so being a late comer I just picked up what I could. I have 6 of them and built them all over 6 weeks
1. Brick Bank. My first Modular build. Loved it. Its a corner building, has a good storyline if you are into that and I recently parked 60142 Money Transporter in front of it which only enhances it. The fact that I got it half price too made it easily the bargain of all the Modulars sets I have.
2. I next built Detectives Office. I loved the alleyway separating the buildings. The pool room and office are brilliant and I was fascinated by yet more building techniques. This shaded the Brick Bank just a little for me maybe because I preferred the colour on the exterior over the White and grey. Despite not being a corner its a great standalone building.
3. Next I built the Parisian Restaurant and having read the reviews of Modulars and this being consistently up there, had high expectations. It really is a neat little build and looks great. My daughter loved the "Art Room" and it certainly brings the market value of any LEGO street up. So many tiny parts for so much detail and as much as I would have liked to agree with the majority, I still preferred the Detectives Office with its brisk built lettering and charm. The Restaurant commits the ground floor to the kitchen Restaurant but not much light gets in and the Kitchen is harder to play with because the whole build is quite narrow.
4. Next I built the Pet Shop. Its cool having 2 separate building on 16x32 plates that you can split up if you like. It may have been worth getting a few of these if on discount to put a row of those brown/orange blocks together. I found the build simpler and less refined than the others and while my daughter loved the "pets" I feel a bigger pet store was sacrificed for the split building. The other rooms were nothing special. This set was nice but bottom of the pile so far.
5 Next I built Assembly Square. Just the sheer size of it is wow in comparison to the rest. The details are stunning and so many different parts. It also had some of the most interesting building techniques so far. I loved the look of the round corner tower with all those cheese slopes perfectly lined up. The was up there with Detectives Office and Brick Bank for fun interiors. Particularly the Characters you can put in the dentist chair, I currently have an Imperial Commander in it with an officer nervously waiting in the lounge
6. Finally I most recently built the Downtown Diner. It really was a surprisingly good build experience. Lots of building on the side techniques and all of the window techniques were interesting. The Diner itself is up there as one of the best "Rooms" of any of the Modulars I love the great big windows that wrap around it letting in loads of light. The gym/boxing ring is good but basic and so is the Recording Studio but is as simple as it needs to be. It would have been better to get more printed plaques and discs to adorn the walls. Also I like the pink Cadillac. You can seat 2 minifigs in side by side in a 6 wide build (They should try this more in City sets) and the pink just adds to the funky colour palette. The Diner was better than I expected and very interesting to put together.
I can't comment on those that I don't have or have not built but I'd rank these 6 as follows form favourite to least favourite (They are all great and its hard to choose to be honest)
Detectives Office > Parisian Restaurant > Assembly Square > Downtown Diner > Brick Bank > Pet Shop