The Brooks saddles most typically fitted on upright bicycles are the B72 and the B66 (the B67 is the same model as the B66, only made for modern seatposts). Having ridden on both, I like each of them very much and cannot say that one is "better" than the other. For me it depends on the bicycle - or more specifically, on my experience of the bicycle.
I had a brown Brooks B66 S ("S" indicates the shorter, women's version) on my previously-owned Pashley Princess, which I rode for over a year. And I recently bought the same saddle for my vintage Gazelle.
I have Brooks B72 saddles on two of my current bicycles: a brown one on my Royal H. mixte, and a black one on my Bella Ciao 3-speed. (I have also owned several vintage B72s, but will leave those out of this discussion).
[image compiled from brooksengland.com]
To get a good sense of the differences in dimensions, I've compiled the specs from Brooks into a single chart. You can see that the B72 is very slightly wider than the B66 models (though a 4mm difference is negligible). The B72 is a unisex saddle and does not come in separate male and female versions; its length is in between the male and female B66. The B72 is lighter and has less height to it, because its springs are tighter and smaller than the springs on the B66. Unlike the B66 models, the B72 is available in black and brown only.
In terms of experiencing the saddles, the main differences are the springs, the shape, and the texture. The B66 has large springs that provide quite a bit of suspension. The texture of the saddle's surface is pebbled. And the edges - including the sides and the nose - are rounded. To me, these features make the B66 particularly comfortable on a bolt-upright, heavy city bicycle. The substantial springs provide excellent cushiness when most of my weight rests on the saddle. The pebbled texture keeps me from sliding around. And the rounded edges make it easier to slide off the bicycle when dismounting - which is convenient when making frequent stops in the city. On the other hand, when I try the B66 on sportier bicycles (with lower handlebars and steeper geometry), I find some of these comfort features distracting: The springs are too much and the pebbled texture is annoying when I am always shifting on the saddle.
In comparison to the B66, the B72 seems like a more versatile saddle to me. The B72's springs are tighter and smaller - still providing suspension, but not to the extent that it interferes with a more forward lean. The surface of the B72 is smooth and the edges are squarely tapered - which I find more comfortable for long distance cycling, during which I slide around on the saddle more, but get off the bike less frequently.
While both saddles are great for upright bicycles, for me the B72 works better on the more sporty variety, whereas the B66 works better on the super-relaxed ones. As they say, your mileage may vary. If you have tried both models and have a preference for one over the other, please share your experiences.