I second TMO, RSRDC, OTC. (My current favorite combo.) Add on an easier AI to begin with (or stay with
) and you're in business. (or substitute TMOwTw - with training wheels - for TMO)
IMO arguably the most realistic as well. (That's perhaps a debate for a different thread, but TMO/RSRD (with easier AI) is about as realistic as a buggy computer game (er...simulation) designed over 60 years after the fact (for 2Gb XP computers) can get!)
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RFB/RSRDC/ISP is great too (do yourself a favor and start sometime after 1943 in career mode. You can always go back to '42 later to get an appreciation of how bad it was!)
When my hunting hasn't been going great with the other mods, I still like firing up GFO (with the stock original campaign) to shoot fish in a barrel. (Plus Webster added a bunch of great single missions.)
When you feel like playing the European theater again (along with the west Atlantic, North Atlantic, Caribbean, Indian, Mediterranean,and even Pacific) in a Nazi sub, go for OM/OMEGU.
Then there's WDAD for British subs, Subron 50 for running around in a Gato in the Atlantic; there's even a Japanese campaign, but I really digress...
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Try 'em all and figure out your favorite. MultiSH4 makes it possible.