The most customizable?

All shipments will have your logo placed on the front of each box, and a customizable insert on the inside of the box.

I know it's customizable and the argument is that "most" accountants use QB. 

Try that calculator I linked you too, it's fairly customizable which I like. 

From reading these posts, I realize the online version is not as customizable, so I’m not going with that.

It is customizable and powerful.

They offer to screen the tenants, where the prospective tenant fills out a customizable application, and they run both background and credit checks.

AppFolioPros: Comprehensive feature set, including online rent payments, maintenance requests, and financial reporting; user-friendly interface; mobile app available; good customer support.Cons: Expensive compared to some other options; may require a learning curve for new users.BuildiumPros: Affordable pricing; easy to use; offers online rent collection, maintenance tracking, and accounting; mobile app available; excellent customer support.Cons: Limited customization options; lacks some advanced features compared to other software.Yardi BreezePros: Affordable pricing; easy-to-use interface; robust accounting and reporting features; mobile app available; excellent customer support.Cons: Limited customization options; not as comprehensive a feature set as some other software.Rent ManagerPros: Comprehensive feature set, including online rent payments, maintenance requests, accounting, and reporting; customizable to fit specific needs; mobile app available.Cons: Expensive compared to some other options; can be overwhelming for new users.PropertywarePros: Affordable pricing; easy to use; offers online rent collection, maintenance tracking, and accounting; customizable to fit specific needs.Cons: Limited customization options compared to some other software; mobile app not as robust as other options.

Its very powerful and customizable.

Not cheap but very customizable for real estate investors.