Yes, weighing solely on margin loan interest rates, IBKR is the best pick I've found. I believe Robinhood might be a better comparison than Ally Invest, if you're optimizing for margin rates.
What will you do with the margin loan? Invest in more stocks and ETFs, creating leveraged returns (up or down)?
What do you plan to invest in? You can also get leverage from certain ETFs or using options.