All jumpseats are comfy as long as it's a ride home.

But if I did have an opinion on the matter;

73-middle is not bad on anything under 90 minutes. Take the covered smoke goggles and place the concave side toward you, then place it on the little ledge to your right. Makes a handy head rest. 73 behind the CA, uh, not so much. I once did that from DEN-EWR. Thankfully the other guy got out for a while.

75-I commute on UAL's, naturally, it's rarely (if ever) the 3 or 4 that had the second seat installed. Not too bad, but when the CA brings his seat back and you gotta go sideways, your feet fall asleep. At least you can stand up and stretch if you want.

319/320-Center is better. I like the H-harness. If you're gonna study your eyelids, just cinch it up. And remember to ask for a cup to put in your lap to catch the slobber. Behind the CA, not bad, not good. Kinda like the 73, anything over 90 mins can be tough. Good thing is it has PLENTY of bag storage.

777-The main seat is a king's throne. It goes up, it goes down, has a foot rest, reclines, has a table that folds out, it's the shizzle. The one behind the FO, it's like your stuffed in a corner.

MD80-Not bad, especially after you figure out that there are foot rests.

727-I once spent 4 1/2 hours in the rearward js. Good grief, what a loud/cold cockpit. At least back there.

CRJ/ERJ-Both crap. At least on the ERJ you can fold the back down which gives you a little more leg room.