Im sure this has been a topic before but I thought id share.  Im a big guy pushing 300lbs and I struggled finding a cushion I was comfortable with.  I can sit on all cushions for about an 30-45 min but then begin to get uncomfortable.  When the birds are doing right it don't bother me but as everyone knows its a rare occasion and sometimes you got sit for awhile.
I have found the air doughnut seat is by far the best seat I have found.  I have bought one from Breedlove air cushions and have been using for couple years now.  It just a inflatable tire tube in sewn in pouch.  Only downfall is that it can puncture.  
I don't work for them or trying to sell them, just thought Id share what has worked for me.  Nothing worse than always being uncomfortable.