As intruder_bass says, 1/144 is probably the best sub scale smaller than 1/72 - you can get Type VIId (minelayer), VIIc, XXI and XXIII U-boats, early and late-war Gatos, and quite a few modern US, Soviet and Chinese subs.
On a smaller scale, in 1/400 you can get a wide variety of U-boats (from Mirage), the Polish ORP Orzel, a British U-class, and various WW2 Soviet types.