For real-time credit card processing consider Authorize.net and Stripe.com - those are pretty much the gold standard in online payment processing right now with Stripe getting very popular. PayPal Payments Advanced is a valid option as well if you negotiate a good discount rate.
If you sell mostly retail, do not forget to offer Paypal, Amazon Pay, and Apple Pay. It's easy to set up and some retailers report 20-30% of orders paid by those methods.
Paypal Credit and Bread (https://www.getbread.com/) will be a good option to offer if you sell high ticket items.
For B2B transactions, consider adding the Net30/PO payment method.
We also have an integration with Punchout available.