What is the best accounting software to use with an outsourced accountant and why?

That's a trick question! I say that because you asked this question in the SaaS section. SaaS solutions are great for outsourced accountants b/c you can (usually) give them access from anywhere (the beauty of SaaS) and you can easily configure the access and apply it to one or more accountants or, for that matter, auditors when it comes time for your annual (or other) audit. SaaS is great for access anywhere and easy user or "role-based" security configuration.

I said it was a trick question b/c there are non-SaaS platforms that can also allow for accountant access. There are a number of SMB accounting solutions that can easily be had in a hosted model such as good 'ol Quickbooks and many other platforms. Hosted solutions allow for access anywhere and most platforms allow for fairly granular user controls. So you don't have to only look at SaaS solutions.