Comfort, riding position and effort are pretty hopelessly intermixed. When you pedal harder, you inherently transfer weight to your legs and off the saddle, this is why racers can get by with minimalist saddles. For example, I'm quite comfortable cruising at 15-17mph on my road bike, if I dial back to 10 mph, the bike starts to be really uncomfortable.
Upright hybrids and beach cruisers are best suited for minimal effort. They have large wide saddles and handle best with a significant amount of weight on the saddle.
My most comfortable bike is definitely my Yeti 575. Part of the comfort though is a combination of shorter stem and wide bars (stable) and the ability of the suspension to basically ignore terrain. But after about 10 miles on pavement, I really start wishing for the road bike...