It depends on your architecture and scale. There is no "best", just "best we've found for this" and "best given other constraints".

This is yet another point where DevOps is not "devs doing ops" but "operations building and deploying with all the tools of modern software development". You need a subject matter expert.

What are you monitoring? Do you care about availability or performance or both? Scale? Do you have services or servers? Do you manage the underlying hardware? Do you need to track which hardware boxes have which VMs or containers?

There are a million questions to answer. One big set of them: what do you dislike about Nagios? Make sure that you don't get those problems with the next one, but also make sure you get something that does what you need as well as what you want.