I've been working on the AS/400 thru IBM i since 1989. Throughout my career I've always worked coding RPG on the IBM midrange platform but also spent many years working Visual Basic, PowerBuilder and eventually Python. In addition to DB/400 I've spent time with Microsoft SQL Server and MySQL with some dabbling in Postgresql.
For those of you touting the IBM i and RPG as the one true answer for business system programming I urge you to look at Python and Linux. The speed of development and ease of hosting make them a great tool for front-ending your secure business data on IBM i.
These days I spend most of my coding time using Python. While we have an AS/400 and IBM i (Power System) at our shop I spend most of my time on Linux servers. My technology stack includes: