The Sea King (with or without the rubber boat strapped to your back) was the worst I can ever remember. It took a year after I stopped flying the things before I realised I had a relatively normal back!
The 206 seat is dreadful, but so many of the older ships have had the cushions reupholstered into so many different shapes, that some are actually quite comfortable
Most Eurocopter offerings still leave a lot to be desired, even the "high back" options only come to the bottom of one's shoulder blades.
The BK117 seat with vertical adjustment is good, even very good, but the standard non adjustable seat is very average in comparison.
Getting away from the basic seat design, is the positioning of the controls: nothing like having a comfortable seat, only to sit twisted sideways to accommodate a collective moving somewhere around your left hip
IIRC, the 222 and the Chinook are about the only ships around with decently designed collectives
