Well, if you are asking what wallet institutional investors use for holding purposes, then they obviously have the following priority.. Non-custodial->Hardware wallet. This means they will obviously go for noncustodial wallets, and then go for cold wallets like hardware wallets. Or maybe air-gapped wallets. And they might not store all the BTC in one place.

Well, if you are asking what wallet they might be using for trading purposes, then they use custodial (exchange wallets) for trading purposes. Mostly these big institutions do OTC trading, which I think can be done directly from non-custodial wallets as no exchange is being used there so they don't need to use custodial wallets.

In general, the most secure way is to use a hardware wallet, or air-gapped wallet, or a paper wallet, but the paper wallets are not considered as more secure than multi-sig hardware wallets.