400k is my favorite distance. It's about the most I can ride on Saturday and still do something else on Sunday.
I ride 80 miles every Saturday during the season, but those are just training rides so not really my favorite distance
100 miles is a bit more than my weekly long ride, but not so much longer that it's anything special
On a 200k there are usually some fast guys that I try to keep up with so I'm dead by the end if not before
I usually still have gas in the tank at the end of a 300k
A 600k is too far for me to ride in one day
Anything longer than that is really more of a tour than a ride