It depends on what you want from the game. Currently SH5 is only three months old, and still needs alot of love and care. SH3 with any of the big supermods is definitely more complete, and at this stage has more to offer. SH4, like SH5, is a better platform for mods and a better game overall, but not as much attention has been paid to it by modders.
So if you want good AI, a complete war, all the u-boats plus dozens of different ship types and all sorts of cool things to see, you want GWX (TGW no longer exists), WAC, or LSH3.
Another option is SH4 with OM and OMEGU. You don't get as many neat things to look at, but you get better graphics, better crew management, better radio management, plus a very good campain and AI structure.
The possibilities aren't quite endless, but there are many.
