I've tried FotRSU, RFB, GFO and just recently, Dark Waters and TMO w/RSRD. FotRSU is my fall back. The overall game play is fun! With the 'More Traffic' mod, it's rich hunting ground. RFB and GFO didn't last long. There were some elements I liked, but not enough to make them 'keepers'.
Repeating what was said here, a great deal depends on personal preference. FotRSU is much more classic with it's campaign where performance earns you medals, rank and quite often a newer boat. RFB wants to keep you in the same boat and grow your crew in experience, which is a nice way to play. TMO appears to have the same mind set. RSRD is very similar to FotRSU campaign wise. They are both well thought out. I haven't dove too deep into TMO to see how the campaign develops over time.
My one issue with FotRSU is the lack of mods. TMO has many more mods available. One mod that I've found that was a game changer was the Optical Targeting Correction by Capn Scurvy. I also have found the ASW in TMO is much more aggressive and intelligent than any other mods I've played. I do have some gripes with TMO. Having preset options is frustrating. You have to start at Hard difficulty for manual torpedo control.
Graphically, TMO and FotRSU are on par with each other. TMO has some environmental improvements that are nice. On the other hand, FotRSU has beautiful sky. A horse a piece. Both are great mods and I’ll continue to play both for their individual differences.
My opinions are free and worth every penny! 