Depends on what you want from "best". Most functional? Get "latest". Or if long-term support matters over functionality, get the latest long-term-support version.
Once you've decided which version is "best" for you on the core application (Jira Core is what matters there), then you'll find that only supports a limited range of Jira Service Management versions, from which you simply select "latest". And read the hardware/supporting services docs for that version to see what OSs it is supported on, what database versions you should be using and so-on.
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