There are several independent international rating agencies that rate financial organizations and instruments. As far as I know, only the ZKB and Bank Sarasin in Basel are AAA rated in Switzerland.

ZKB is guaranteed by the Canton of Zürich, which is financially very strong. Not all Swiss Cantons are equally strong. Some are burdened by public debt. Nevertheless, it is inconceivable that a Cantonal bank, essentially "nationalized" would ever be allowed to go down.

ZKB has a good network in Zürich, Their systems, services and products are good, albeit offering lower interest rates. The ZKB profits flow back to the communes in Zürich. Those revenues help lower taxes in ZH.