Welcome aboard!
All of the super-mods have their own flavor. You'll just have to sample them yourself and decide which one you like best. I don't think you can go wrong with any of them.
I've played both TMO and RFB(Both with RSRD campaign mod) and both are excellent, although I lean more toward TMO. Ducimus(the creator of TMO) designed his mod to push you toward acting like a real submarine captain would. Some items might not be strictly "realistic" but they may make you, as sub commander, behave in a "realistic" matter, if that makes sense.