Wear ear plugs, bring water, and definitely bring a book. If we had ipods then, then I would highly recommend noise-canceling headphones.
Best IMO are KC-135; they have a cargo area up top which is downright comfortable. If you butter up the boom operator, you can watch your jets (F-16s in my squadron) getting refueled during the trip, which is quite cool and the view from the boom operator’s seat is simply stunning. But like all military transports, sound insulation isn’t that good, and you end up not being able to do anything except maybe read a book if you’re lucky. Also quite chilly as the atmosphere control ain’t that good. Of course, you’re loaded on top of a flying gas tank, so you try not to think about the vast amount of explosive jet fuel you’re sitting on 
I spent 17 hours in a C-130 once, on the nylon jump seats, packed in like sardines. I had a rash on my ass that took 2 weeks to go away afterwards, and was deaf for a day and a half. I’m sure that’s where most of my hearing loss came from, that and working on F-16s. Shoulder to shoulder if it’s full up; if it’s really full up you’re not only shoulder to shoulder, but between your knees at a 90 degree angle is another bloke’s knee - forget about stretching out. You get real close to your mates real fast. And they’re farking slow, too boot. Second for “it beats walking” but only just.
C-141 are also quite slow, but faster than a C-130; still loud and cold and uncomfortable and crowded with tiny seats designed by Torquemada and bought second-hand from Aeroflot.
Probably the best was a C-9 Nightingale - it’s basically a Gulfstream IV without all the bells and whistles, acts as a fast medevac plane with lie-down cots. We had a small unit going to the Paris Air Show one year and got to ride on one of those - it happened to be available and going the same way as we were, or else we’d be on a bus from my base in Germany.
I did an incentive ride in an F-16d once; I would imagine they had it slightly worse than we did in C-130s on long-haul, as they’d spend hours not being able to move and getting in-air refueling and pissing in a tube. At least we could stand up once in a while.
Never flown on C-5 or C-17, but I’ve heard the improvements are vast in both speed and comfort.