As for which planet rules the chart, I am suprised that no one has mentioned the planet which has the most essential and accidental dignity, the planet that has the most innate strength coupled with the most innate power to enact that strength, surely such a planet is a front runner of our astrological make-up.
The ruler of the Ascendant is the Ruler of the Ascendant, and not the ruler of the chart, just as the ruler of the MC is the ruler of the tenth house and not of the entire chart. The reason the Ascendant is given such precedence over any other angle is in part because we are all so helplessly egotistical. The ruler of the Ascendant has a particular importance of course, as it is the planet which presides over the self in all of us, and as our conciousness is almost always situated in that self, the ego takes over and gives the ruler of the Ascendant more credit than it is due and considers it the ruler of the life, when the life is so much more than 'I', as the other eleven houses hold testimony to.
If there is such a thing as a planet that presides over the entire chart, the entire life in all it's aspects, then such must be the planet with the greatest essential and accidental dignity - because it makes so much sense.
If I have Venus combust in Aries in a cadent house blighted by harsh aspects from unfavourable planets, and Venus is the ruler of the Ascendant, is it the ruler of my chart, my life. I would hope not, because considering it as such would go a way to manifest it as so, it is the ruler of my perception of life as situated in the self, not of the life.
If I have Mars in Scorpio conjoined with the MC receiving favourable aspects from agreeable planets, and as such is the most essentially and accidentally dignified planet in my chart, but rules not the Ascendant, but yet is the best planet I have to work with, then does Venus or Mars preside over my life? I suppose that it is a choice, because we cause to manifest wherever we direct our conciousness, but I would opt for the planet with the greatest strength to be my representative in this life and act accordingly.
The Lord of the Geniture/Birth, is the planet that shows the highest possible expression of ourselves, it is our guardian spirit whom we should endeavour to act in principle of in any situation in order to manifest our destiny. This is a planet not determined as our horoscopic representative based upon beaming in upon the realm of one twelfth of Life, but it is designated based upon it's strength of dignity outweighing all others in the chart. It is our destiny, the strength of our horoscopes and the stalwart of our lives.
The ruler of the chart, indeed, the Lord of the Geniture, so called as it is the president of our incarnation into this world, is the planet which has the greatest dignity in our charts, and through which we may manifest our destiny.
If our ego's are what our life is consumed with, then the Ascendant ruler will be the ruler of our lives, for anything which spans the entirety of our twelvefold manifestation in existence will not get a look in.
If it is our money and possessions are what our life is consumed with, always or just for today, then the second house ruler will be the ruler of our lives, always or just for today, and again, we act accordingly. The benefit however, of seeing the planet with the greatest dignity as our guardian angel, our 'ruler', is that we endeavour to attach ourselves to a strength that transcends all of these compartmentalised boundaries, a ruler of all rulers, and indeed, the house which is ruled by the Lord of the Geniture, may be the twelfth of our existence that provides the most direct route to the Whole, so perhaps we might better consider this house cusp as more important still than the Ascendant in many respects.
The ruler of the Ascendant as ruler of the chart, and inevitably of the life, means that some have the blessing of wonderfully situated planets to revel in while others are relegated to the reject bin.
The consideration of the Lord of the Geniture as the ruler of our chart is optimistic to say the least, and a manifestation of destiny at most. This is because such a planet always represents the greatest strength and the highest possible given that horoscope, which surely is what life is about, bringing to fruit our most optimum potentials.