Here's my thoughts on Flak guns... personal opinions only, of course... other folk may disagree!
There is the VIIC/4 "U-Flak" conning tower. That gives you three heavy Flak platforms, but is limited to the VIIC, VIIC/41 and VIIC/42 boats only. It also prevents you using a deck gun.
If you want to keep your deck gun (I do!), then for the VIIC, VIIC/41 and VIIC/42 boats, you can install a VIIC/3 tower, which gives you two light and one heavy Flak platforms.
For the IXC and IXC/40 boats, you can use the IXC/3 tower, which has the same capacity.
The best Flak to put on a light platform is the 2cm FK C/38 Flakzwilling (available July'43), because of its stable platform and high rate of fire.
The best Flak for the heavy platform is the 3.7cm FK M 42 Twin (available Dec'43), because of it's high rate of fire, excellent damage ability and
continuous reloading. It is better than the 2,0cm Quad, because the Quad is not properly modelled in the game and has a slower rate of fire than it should have.
If you are using a U-Flak boat, 3 x M 42 Twin is pretty awsome, but the M 42 + 2 x C/38 Flakzwilling option is also good and you can still mount a deck gun.
For more information on Flak guns, see
http://www.communitymanuals.com/shii...line#Flak_Guns