There are a few factors to consider:
1. Safe and Liquid
You need to ensure quick access to the emergency funds, which rules out investing the money in stock market. Why? Because what if the stock market is at a low point, but you need access to the fund?
2. Avoid Costs
Due to consideration above, emergency funds are best to park at savings or money market accounts. It's important to pay attention to monthly fees or minimum required amount requirements. You should look for $0 monthly fee accounts and as low as possible minimum balance requirements.
Here are a few good options to consider, they all offer at least 1.05% annual rate and no or low minimum requirement: