The best king will be the one who has the best support staff.
When you're a king, there are so many matters, many of which so trivial that a human cannot pay attention to them. A King shouldn't try. He should hear of a barbarian horde in such and so, and nod to a Major, who wordlessly marches his men out and deals with it. Ideally, a King wouldn't even hear of the barbarian horde until either it had already sacked a County Seat or more / been dealt with.
Everything is done by staff. Staff do everything.
They should be paid fairly for their competence and to ensure loyalty (coups aren't cool).
Second good idea for a king: Re-Invest.
You hold many lands and make a TON of money. You collect a ton of taxes. Operate a government with surplus, and spend on an as-needed basis through the years end. End with a full coffer and some extra bags of gold laying in the hall.
Then SPEND that gold on something. Something good. Something useful. Something that adds value to the kingdom and will in the 5 year plan result in measurable returns on investment. A quality toll road. An outpost. Patrols to clear the road of highway men. -
Invest in a vineyard. The finest in all the kingdoms. Sell its vintage to foreign lords for a hefty profit. - Invest in a drydock. Create merchant sloops that can outrun any blockade and get your cotton to market in London. Buy the textile mills that make the sails and through vertical integration pay no-one elses' profit margin from timbers and fibers to finished vessel. - Invest in a publicly owned and managed natural reserve. Staff it with foresters and wardens to sustainable grow timber for your nation, and provide stocked hunting grounds for locals and vacationing lords alike.
Be an Industrial Capitalist King. Read how the steel barrons made it work, and how the mill riots their abuse caused killed thousands.
Then, don't abuse the people who work day and night to ensure you're the richest man in the world.
A dollar is worth nothing until you spend it.