It is hard to conclude which IDE is the best for development, as everyone have their own preferences in this matter. Most IDEs have their strengths and weaknesses. We officially support a number of IDEs/compilers, and you can find tutorials and blogs helping with setup of alternative IDEs.
Personally I use Keil for most projects, as this is the simplest to setup and the one best integrated with our SDK, but it is expensive if you exceed the 32 kB project size. If you are looking for a free alternative, Segger Embedded Studio (SES), which is free for non-commersial use, is a great alternative. Another alternative is to use GCC with Eclipse, as described in the tutorial Development with GCC and Eclipse, but this is often more work to setup and get working with debugging etc.
I prefer the open source route, even though it may take more time to set up. However, the benefits are worth the extra effort and time, especially when you consider that any learning curve and setup you have to go through can be used for other development boards/kits and from different vendors.
IMO, I like to use NetBeans as my IDE of choice (and I've been using it for about 10 years now for many different embedded platforms). I've written a comprehensive tutorial on how to set it up for the nRF52840 Preview Development Kit on a Mac.
