I have bikes made of titanium, exotic aluminum alloys, carbon fiber, and stainless steel, but I have put old-school leather saddles on them. Why?
The saddle on my first serious bike (Windsor Pro Columbus Frame with Campy Nuevo Record & Cinelli components) was a used Brooks Pro. At the time I didn't know better so I didn't realize how lucky I was. I found out with my second bike (Ron Cooper Reynolds 531 Frame and most of the components from the Windsor Pro) when I bought a shiny new Brooks Pro Saddle for it. It was hell breaking the saddle in, but eventually worth it.
Over the years for the sake of weight and fashion, I gave up my Brooks saddles for plastic ones with either foam and/or gel padding covered by either plastic or leather. A few of them have been acceptable for longer rides and one qualifies as a device for slow torture.
The Terry Men's Liberator Gel was certainly one of the best in the plastic and paddling group. My opinion is that is deserves the good reviews. An honorable mention needs to go to a very inexpensive Serfas Deep Groove Saddle that I had on a road bike for several years. It was wide enough, with a smooth cover, nice shape, narrow horn, large cut out, and probably too much foam padding.
The Specialized Avatar Gel Saddle (155mm width) is hands-down the absolutely worst saddle I have every experienced. I know they put good science into the design and the saddle has received good reviews, but, after riding it for just a few hours, it would create silver dollar size sore spots where my sit bones contacted the saddle. I am not talking about a little bit of discomfort, but bad enough to keep me off my bike the next day. Ironically the less expensive model, the Specialized BG2 Sport Saddle (150mm width), I found more comfortable which also provided a clue as to why the Avatar was just so bad for me.
Third on my most comfortable saddles list is a stock Brooks Pro (160mm width) now on my hardtail mountain bike. It was tough breaking it in, but with frequent applications of Brooks Proofide Leather Saddle Conditioner and lots of miles it is now a pleasure to ride. The more upright riding position I believe keeps the pressure off things that shouldn't have pressure on them, so no cut out or slot is not a problem.
Taking second place on my all time most comfortable saddles list is the MCM Selle An-Atomica Titanico LD . It is similar to a Brooks saddle, but with a slot or cut out in the leather that is nothing short of a brilliant bit of elegant design. This enhancement allows the leather to more easily flex with the rider's movements which signficantly improves comfort. However the saddle gets a few points off because the rails are not as robust as I would like and the craftsmanship is not up to par with Brooks. I also found the saddle tended to flare out and rub my thighs until I drilled and laced it. This actually worked out since the saddle with the addition of the old-school lacing looks great on my fixie. One other thing I have to mention is that MCM Selle An-Atomica has great customer service! Their innovation in leather saddle design and its benefits are described in their white paper titled Bicycle Saddle Design Fundamentals, Research & Development 2002 – 2006
Number one on my list of most comfortable saddles is my Brook Pro modified by MCM Selle An-Atomica with the addition of their LD slot cut-out. This saddle was a pleasure to ride right out of the box and it has only got better with use. Only a few months after purchase I rode this saddle on the 2007 Bicycle Tour of Colorado and I honestly didn't even think about saddle discomfort the whole trip!
So what makes a saddle good or bad? Well my experience is that most saddles that fit the width of your sit bones are comfortable for short rides. However one needs to put a few hundred miles on a saddle over a few days before he or she can appreciate other aspects of a saddle's design that affect comfort.
My experience with rides extending of 50 to 100 miles is the amount of comfort the saddle itself contributes to the experience (This is assuming its height, pitch, and set-back are set properly!) depends on the following 5 characteristics roughly ranked by descending order of importance:
Width - The back of the saddle needs to be just wide enough to accommodate the spacing of your "sit bones" for your given riding position. Both the taper or shape of the saddle and its horn need to be narrow enough to not interfere with leg movement. I am a Clydesdale that needs a minimum 155mm width, but smaller framed people may need something a lot narrower especially with an aggressive riding position.
Low friction - The saddle's surface needs to be relatively smooth and slippery. The friction between your shorts and the saddle should be considerably less than the friction between your shorts and skin. Simply put, your shorts should stick to you not the saddle.
Weight distribution- The saddle should deform a bit around the area the supports the sit bones in order to better distribute the load.
Flex - The saddle needs to give a bit in response to shifts in position and body weight during pedaling.
No pressure points - The saddle is designed so that it does not put any pressure on nerves or blood vessels that may lead to numbness or worse.
So what makes the Specialized Avatar Gel Saddle so uncomfortable for me to ride? It is wide enough. It has gel padding to distribute the load and fair amount of flex in the plastic body. It has a generous slot down middle to avoid pressure points. Well, what makes it miserable for me to ride is that it fails my criteria number #2. The back of the saddle has a suede patch stitched to either side right were my sit bones go. These patches held my bike shorts in place while I moved within them basically rubbing off skin immediately under my sit bones as I rode. The Specialized BG2 avoids this flaw by using a smooth cover, but this saddle is a little narrow for me at 150mm and I thought it might have a little too much flex.
Where Specialized's Avatar design fails, MCM Selle An-Atomica's and Brooks' designs succeed by using a single piece of polished leather. This characteristic is so important to my comfort, I keep my leather saddles shiny by polishing them with Brooks Proofhide and a soft cloth as part of normal cleaning and maintenance.
So Brook's craftsmanship and MCM Selle An-Atomica's brilliant LD slot design combine to yield a near ideal solution for me.