I recommend the following link on the Fleet Submarine website. It lists all US subs and has a quick link for each class that lists basic specs for each sub class. The link is:
http://www.fleetsubmarine.com/sublist.html
A basic listing of the sub classes in the game (ranked in order from earliest commission date to latest commission date) is:
S-class (1920) 4 Bow Torpedo Tubes
P-class (Porpoise Class) (1935) 4 Bow TT / 2 Aft TT
Salmon class (1937) 4 Bow TT / 4 Aft TT
Sargo class (1939) 4 Bow TT / 4 Aft TT
Tambor class (1940) 6 Bow TT / 4 Aft TT
Gar class (1941) 6 Bow TT / 4 Aft TT
Gato class (1941) 6 Bow TT / 4 Aft TT
Balao class (1943) 6 Bow TT / 4 Aft TT
As for the best US sub I vote for the Balao class because it was the deepest diving and newest sub class built in the war. I am sure each skipper will have a favorite to play in this simulation so pick the one you like and enjoy!