Best is very subjective. JIRA already supports mobile interfaces.
Cloud also have JIRA mobile connect preinstalled
What is missing for you?
Norman is absolutely right - "best" is subjective. Your comment suggests that you want a generalised "just do stuff" client, which needs to do the basics. That rules out some clients which are highly focussed on one aspect, but doesn't really narrow it down. Your best option is really "Try the available ones and see which suits you the most".
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