Hand of the King is an extremely important job. Their role, officially, is to advise the King on all matters great and handle all matters small. They oversee the Masters of Coins, Whisperers and various other Small Council members to keep the kingdom running smoothly.
Such would be the job if things were easy. Alas, in Westeros, things never are. The Hand is also supposed to sniff out traitors, to manipulate or convince the unwilling, and to secure the safety of the King as much as their success.
Some people make good Hands because they’re highly organised and run the Seven Kingdoms efficiently; others because they’re an iron fist around the Iron Throne.
A few characters here served as Hand to various rulers; their performance overall is what determines their ranking though. Just as it would be in real life, consistent, steady Hands are better than erratic ones.
With female leaders in the ascendancy in Westeros during the era the show is set, there’s some Hands of the Queen in the mix too. As it’s an important role, some of the show’s biggest characters have filled this role, although they haven’t all shone.