[Sledgehammer427]

RFB used to be my go-to mod. It was well put together and definitely the option if you want a stupendously realistic experience, though some of the graphics (mostly the environment) can be seen as lacking.

TMO used to be the gold standard of SH4 super-mods. It still kind of is. I've been playing more TMO lately because I like the concept of challenging escorts, and everything else involved is good to the point I can't not enjoy my time playing it. Ducimus put his heart and soul into TMO and it shows.
EDIT: one thing I forgot to mention with TMO and the reason I've been sticking to it is that Duci put together some great campaign additions, like the option of a "New Command 1942/43" that will give you the opportunity to sail a Gato or Balao as early as historically possible. To say I haven't tried to port these amazing "intro" campaigns into RFB/RSRDC is a lie entirely. Its the main reason I keep going back to TMO over RFB.

RSRDC is purely a campaign mod, Lurker spent a lot of time adjusting every layer of the taskforce, convoy, single ship traffic files and made it a much more realistic experience. You can usually assume that if you've spotted a ship somewhere in the game with RSRD enabled, Lurker found a document somewhere that said that type of ship was there. As its a campaign only mod, its been adapted and ported to RFB and TMO. His Atlantic mod, Operation Monsun, is also worthy of mention.

FOTRSU is still in testing, and to me, having played it a little last night, feels mostly complete but its still in a public beta. We are the guinea pigs, but altogether it feels good. I can't comment more on it as I've only put an hour or so of time on it.
ANOTHEREDIT: There aren't many mods that are compatible with FOTRSU because the FOTRS is still in testing and modders may be willing to wait until the mod is complete before trying to add their stuff onto it, instead of doing it now while things may be subject to change. Another reason, maybe, is that a ton of mods are getting to be 8-10 years old now, and the modders behind them may have moved on and forgotten all about SH4.

Your mileage may vary, scanning the readmes and manuals of the mods in question should give you the best overview of the mods you are most curious about. Altogether, you can't go wrong with any choice you make.