QuickBooks
This is the most standard and safe pick of all startup accounting software. It's almost necessary for payroll processing and integrated merchant processing. Additionally, almost every bank integrates with it and most accounts use it. You can't go wrong.
Xero
Xero performs all the same functions as quickbooks, but has some different features. For example, Xero doesn't automatically add late fees but it sends payment reminders. It's a little simpler to use, so if you're doing your own accounting, this could be a better option.
NetSuite
If you haven't raised a Series A, then read no further. This is for the soon to be unicorns. For example, Brex started on Quickbooks but quickly moved to Netsuite because of its robust accounting approval flows critical for the types of financial control and IT audits it regularly completes.