What is the preferred ATM company to use in Croatia?

Try a bank-owned ATM. Always choose the local currency for the withdrawal not your own ("Dynamic Currency Conversion") which would impose a completely unnecessary fee.

When I used ATMs in Croatia a few years ago, I was never charged a fee. I didn't care who owned the ATM as long as it was a bank.

Avoid Euronet ATMs, they are essentially rip-offs and it sounds like you came across one. There are no Euronet ATMs in Slovenia, but they are common in Croatia.

I've also encountered no unusual fees with bank ATMs in Croatia. If you're driving and headed to a rural area such as Plitvice, considering stopping in a town along the way to use a bank ATM as bank AMTs can sometimes be scarce in rural areas. For example, I stopped in Karlovac and used a bank ATM there on my way to Plitvice.