My daily use is a side cutter from Walmart, though not their private label brand. I have worn the printed name off the handle, but it might have been Faberware, though I do not think so. It has done yeoman service for years. I have two more of them in the deep stockpiles.
I have had good luck with all of the Official Swiss Army Knife can openers on Victorinox, Wenger, and Aitor versions. They are spread out among my gear in various kits, EDC bags, and LBE systems.
I always hated using a P-38 as it was hard to use, hurt my fingers something fierce, and due to some hereditary bone structure stuff, using one just literally would cause my fingers to lock up. I can still use a P-51, though it does hurt. I have several of them on EDC keyrings as well as some stash keyrings. I usually buy the actual military supplier-made ones by the dozen. I could not tell you how many of them I have given away to homeless people I see trying to get a non-easy open can open.
I may have used a pocket knife once, to open a can, but my dad saw me and told me he better not ever catch me doing that again to a good knife. (And not in the sense to make sure he just did not see me, but in the sense of do not do it.) I have used a Dremel with a metal cutting blade as well as a couple of times with a grinding wheel. And once a cordless 4 1/2" grinder. That was almost a disaster as it opened that can really, really, really fast.
Though I did do the Dremel and the grinder, it was to prove to someone that they did not have to have a can opener can opener. That there were lots of ways to open a can, especially if you were any kind of tool guy, especially if you had power tools. Not that I would not do it if that was all I had, it is not something I do on a regular basis. Gotta say, if you do use a grinder to open a can, be ready for a food bath, flying can lids, flying cans, and if there is anyone else around, some loud cursing.
I never tried a file, though I know it would work. So does concrete, as long as it is not a slick finish type. Have to be real careful there, too, as the can is inverted (unless you find an overhead concrete slab somewhere) and if you keep everything really even, the can lid is suddenly no longer attached and when you try to tip the can upright not everything wants to stay inside. So be careful if you do the movie thing of opening a can by rubbing it on concrete. It does really work, and quite well, but you do have to watch carefully and have a way to finish opening the can when you just begin to see a bit of moisture oozing out of the edge.
Pounding cans with rocks, metal bars, baseball bats, 2x4s, sledge hammer... That just does not work very well. Sometimes you can get the can open, but nothing inside is still inside and in condition to eat. Hatchets and axes work, too. And if have the aim of some mythical axeman, you can open the can and still have food in it.
And just so you know that last set of options that I described I learned about from a pair of friends that were basically idiots. Though that does kind of denigrate idiots unfairly.
Anyway, get a decent side cutter to use, and three or four more once you know it is a good one, to put back, and get a double handful of P-51s and scatter through your stuff so you will always have one somewhere close.
All true, but still pretty much my opinion.