I suppose you are on the Mogami CL/Myoko, trying to get some levels.
For mainly AA
I recomend those 2 x 3,9"/65 Type 98 D. I actually used them myself a long time ago while lvling my AA gunners.
Also those 5"/50 Type 3C D are a good middle way. They have same 38 degree golden angle as 2 x 3,9"/65 Type 98 D. It will teach you to snipe planes. Still they leave you with good anti ship dps.
The tripple 6,1 are for mainly shooting at ships, for wich they work very well. They have the longest range, but also longest reload. If you are good at sniping, these will work as well.
As always i recomend going for 3/4 AA shells, and rest heavey HE. This is the IJN CL/CA AA setup for GBs. For T slots go for HH.