Bulk standard building wise I try and have a mix. Tall, short, wide, thin, T shape, L shape, H shape. My collection includes the original AT polystyrene, 2nd ed card, EA ruins, Battlescene and waiting on the ones from Wastelands Kickstarter.
Depending on what sort of table you are going for, I did some sketches for a full city board (6x4) and assuming a building size of 10sqcm, you would want about 40 to give reasonable coverage (also depends on large structures like cathedrals). Something like this:
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City Layout.JPG [ 55.61 KiB | Viewed 4364 times ]
I agree with Legion that a flat roof is useful but not required, especially if it opens up more interesting designs.
We seem blessed at the moment with several people making decent 6mm buildings, but most fit into the tall, square gothic look. Again, like Legion says, for me it would be the interesting shapes that catch my eye now. I am especially looking for 2 things at the moment terrain wise (not specifically buildings):
1 - Industrial, refinery, chemical plant type stuff. Distillation columns, piperacks, storage vessels/tanks/spheres, flare stacks, gantrys, cranes etc.
2 - Rail/train. A decent and cost effective rail set including, track (straights, curves, points), locos, wagons, carriages, loco sheds, stations, stock yards.
Matt