B&M clamshell restraints are great. The whole seat experience is great... very comfortable and they even have armrests! But really, the only time I use them is when I'm waiting for them to dispatch the train.
Millennium Flyers. Just overall the best woodie trains there are. So comfortable, they are like living room furniture.
And I also really liked the seats on Millennium Force. I thought they were supremely comfortable.
I think the least comfortable seats I have ridden in were on Anaconda at Kings Dominion. For some reason, they didn't have any padding at all! Just solid plastic... pretty lame.