I'm partial to my Brooks B-17 Imperial. I have a B-17 on my singlespeed, a (not so gracefully) home-butchered B-17-turned-Imperial on my utility cruiser, and a true Imperial on my brevet bike.
The lacing on the side skirts of the Imperial keeps the center top section from sagging, and the cutout helps prevent pressure on the perenium, which (for me) eliminates any pain or numbness issues even when I'm on my bike for 20+ hours.
The WTB Rocket series, Selle An-Atomica (similar to B-17 Imperial) and Selle SMP saddles are also continual favourites among the club I ride with.
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"I feel like my world was classier before I found cyclocross."
- Mandi M.