If it's an opinion you want, well... but I prefer SH4, the US Fleet Boat side in the Pacific, but you can play UBoats in it too. There are all sorts of mods available for it, including a few for the UBoat side. It's got better graphics than SH3, and isn't as broken "out-of-the-box" as SH5, though there are some really cool things in SH5 (Uboats only). Wolves of Steel and Sober's Bonus Mod are the only two ways to go in SH5 (my opinion), though as I always joke, I can't get up and down the ladders in the sub... SH3 (Uboats only, basically) has the broadest selection of mods, and the most mods that work together. Really nice stuff in GWX, LSH & WAC...
You could probably do some research on YouTube and watch some of the videos for each, and get an idea of what you like visually in them, and browse around here in the respective SH forums for what you like in game play. Are you more of an auto targeting kind of person for the torpedoes, or manual targeting? Do you like being able to hit shortcut keys for instantaneous crew response, or do you want to have to "speak" to a specific individual to get something done on your boat?
I understand completely about misplacing the game disks. I couldn't find my old SH3 nor my original SH4, and purchased other copies myself. Steam has versions of them available, but preparing a Steam version for modding can be bothersome, especially SH5. Amazon has these in the US download store (SubSim Store links)
Silent Hunter III
Silent Hunter 4: Wolves of the Pacific - Gold Edition
Silent Hunter 5: Battle for the Atlantic - Gold Edition
There are, of course, other sites that offer all three, including some with the DVD disk, though they are definitely getting difficult to find.
