I have read some of your prior posts; and recognize you are an advanced astrologer, so if you HAD to choose one planet, which one? Or, which planet in your chart is responsible for your own unwillingness to choose?
It varies based on the each individual birth chart - and some charts won't even have one "most important planet" - they'll have two or even more. It's not an "unwillingness to choose" - but a willingness to delve deeper into a chart than saying "The Sun is the most important planet in any chart."
In some charts, the Sun might well be the most important planet - if it is in a day chart, high in Essential Dignity, has Dignity in the rising sign/degree, and is in an angular or succedent house.
In a night chart, the Moon would take precedence over the Sun if it is well placed or not debilitated, and has dignity in the rising sign/degree.
Normally, if the Domicile Ruler of the rising sign is well placed by both essential and accidental dignities, that would be the most important point in the chart. If not, then final/multiple dispositors and mutual receptions by Domicile would be considered.
As an example, take my chart (it's in my profile, if you want to look at it). It's a Night chart, and my Sun is placed in Aquarius, Peregrine, in the 2nd House. It has no Essential Diginity and therefore it is not a comfortable placement.
My Moon fares somewhat better. She is the Sect Light in a Night Chart, in Virgo and therefore in Triplicity - but she's in the 8th House in Regiomontanus and the 9th in Whole-Sign houses. So not a good placement in the 8th, and Cadent if considered in the 9th.
Mercury is the Domicile ruler of the Sect Light, but he is in Detriment, Fall, and Peregrine in Pisces. Not much help there. Since the Sect Light would be considered cadent by the Hellenists, the Oikodespotes would then be based on the Light out of Sect. We'll get back to that.
Looking at Essential Dignities, my highest-ranked planets are Jupiter in Sagittarius, in Domicile and Triplicity (+8 - according to Lilly), Mars in Capricorn in Exaltation and Term (+6), and Saturn in Capricorn in Domicile (+5).
However, Jupiter is cadent in the 12th House - so although he's comfortable, he really can't operate very well.
Mars in Capricorn, in Exaltation and Term, is very strong and angular - a very good candidate for the strongest planet.
Saturn in Capricorn is within a degree of the Capricorn ascendant, in the same Term.
The Almuten of the ascendant is a tie between Mars and Saturn.
However - remember what I mentioned about the Sun determining the Oikodespotes in a chart where the Moon as Sect Light is Cadent? With the Sun in Aquarius, Saturn becomes the Oikodespotes. And also the Kurios by virtue of it's position in the Term of the ascendant and other factors.
So Saturn turns out to be the most important planet in my chart, followed closely by Mars, then Jupiter, then the Moon. The Sun isn't really all that important in my chart.
I hope this example clarifies things for you a bit.