[CCIP]

Think of what you'd most like from the game, too!

How important are things like graphics, variety of enemy of units/traffic, sound/atmosphere to you? What do you like as far as difficulty or pace of the game? Do you like to spend a long time executing deliberate attacks that might result in nothing, spend a very long time on patrols, or do you want to find and sink more ships quicker? Do you have time to spend hours evading destroyers every time you attack a convoy, or would you rather resolve it quicker? Etc. etc. I think that might help you find the right version!

My advice is to stay away from arguments about "realism" because all of the modern SH3 mods are very realistic in their own ways - and all offer lots to the player to immerse themselves in the SH3 world. "Realism" is a very subjective thing in a video game and what works for one player doesn't work for another.
I personally prefer the NYGM flavour because it fits with my preference for slower, more methodical gameplay, high difficulty and lots of player involvement, and having to really work for my tonnage and survival, often coming back with very modest results. Maybe it's because, more than anything, I'm interested in the "slow, boring" work of the average U-boat at sea rather than trying to live out my fantasies as the greatest ace of all time. Makes the success all the more sweet when it comes. But there's a lot of great play to be had in other mods, too!

As a starting point, a GWX-based install might be pretty good! It's a bit dated now since development on that mod has long stopped, but lots of added mods still support it. It's almost like an unofficial patch to SH3 by now.
Then you can see what you like and/or don't like about it, and go from there - either adding things to it or switching to another "supermod" base (like NYGM or WAC).

Whatever you choose, be sure to learn to use JSGME for installing/switching mods; get SH3 commander; and get h.sie's hardcode .exe fixes, all of which add some very new and exciting stuff to SH3.