The best case is the one that makes it easiest for TSA employees to put the bike back in. Period.
I've flown *a lot* with bikes over the last several years, often with more than one bike on the same trip, and every single mishap I have had was the result of TSA re-packing the bike incorrectly or simply not even closing the case/bag all the way because they couldn't get everything back in the way it came out. If you have to sit on the case to get it closed, rest assured TSA won't. If you have to struggle to get the fastex buckles to snap, rest assured TSA won't.
Corollary to this is that - again, IME - TSA can also be counted on to not get everything back in your case if you give them that option. I've lost shoes, tools, sunglasses... a fair bit of stuff. The fewer pieces you have in the box, the less they have to screw up or leave out when they put them back in. If you are going to put extra stuff in the case, put it in a bag inside the case.
Needless to say there is absolutely no recourse when stuff goes missing or gets broken. None at all. Even when I picked up a clamshell hard case from the baggage carousel with the handlebars sticking sticking out the gap between the halves, with the halves taped together and the straps tied in loose granny knots, and one shoe bouncing down the carousel, the other long gone, I was s**t out of luck.
I think the absolute best case is something that you can fit an entire, assembled bike in that zips open and shut. With a big g-damn zipper.
Get as close to that as you can.
Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
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