Whatever it will shoot reliably. Some folks are having problems with Tulammo, steel cased (91 grains) in the black box with red writing and made in Russia. The Tulammo in the white box called brass maxx, made in Italy, brasscase and 95 grains seems to be fine though.
My .380 Thunder Plus has problems with Winchester white box FMJ. It has a flattened nose that causes the gun to FTF a lot. I tried a few other brands of round nose FMJ and they work fine. I even used some Brown Bear that has a very copper colored brass case and it seemed to work best of all. No FTF or FTE issues, but it was very dirty. I have not tried any hollow point defensive ammo yet. I also have a Thunder .22LR and it's also very picky about ammo. It does not run on the cheaper bulk .22LR like Federal or Winchester. It only likes CCI Mini-mag or CCI Stinger.
Just picked up a standard 7 round Thunder .380 this week and hope to get it to the range this weekend and see what it likes to eat.