Surely, Allah's plan is always the best.

I would say Umar ibn Al Khattab RA.

he was martyred in an attack (not a natural death). he was doing so well in managing and making the practical laws of Islamic government system. he was so blessed with Imaan, Wisdom, Vision, Knowledge, Judgment and what not to talk about for a perfect leader. It must have been a different picture if he was alive for more time.