I voted the DD, it is the backbone of every defensive or mixed fleet.
The CC imho is the least usefull unit, but still has some use in large fleets. The question always is, why buy a CC and a DD with 20 IPC when the same money can get you a BB. Well, the answer is fleet volume.
When playing Dice, you can get lucky or unlucky. But, no matter how lucky you get, one BB will only get you one hit (in att or def) while one CC and one DD, if you are lucky, will get you two hits. So, for every second BB I buy, I prefer adding some pure volume in my fleets by bying a DD and a CC.
Another equivalent is to buy 2 SS and one DD, but when def is a primary concern and volume is needed, 2 SS are no more than fodder, given the attacker will have a DD (or else his air hits won t be assigned on the SS, and they will be useless even for fodder…).
The optimum setup for a mixed fleet (for example a US2 pacific fleet aiming for south pacific in US3) is 4 SS, 3 DD, 1 CC, 2 AC loaded, 1 BB, 2 TT (carrying 4 inf). Total IPC cost 82 (buy US1 2 AC, DD, SS, US2 1 ftr, 1 TT+1 inf, 2 SS, DD).