Technically, I believe you have to see which planet is eventually ruled by itself and not by another planet.
For example: you have sun in virgo, which is ruled by mercury. Mercury is in libra, which is ruled by venus. Venus is in scorpio, which is ruled by pluto. Pluto is also in scorpio, which means that pltuo is ruled by itself and at the top of the chain.
Problem is with anyone born between 1984 and 1996, they will have pluto as their co-ruler no matter what, and I consider that somewhat of a weakness in that system.
I generally go by what I see. For example, according to the above system, Pluto is my ruling planet. Yet pluto is, first of all, not even a planet anymore and it exerts very little influence in my chart. The biggest influence in my chart is where most of the activity is. Which is my 1st house and Sagittarius, with 4 planets in them, and between the planets there (and the moon, which opposes in the 7th house and Gemini), most of the activity in my chart. Also when you look at my personality and behaviour you'll see a clear 1st house influence, even over the Sagittarius influence. I'm very impulsive and hardly think before I say anything. Typical Aries behaviour.
Technically speaking, Pluto is your biggest ruler, but both Saturn and Uranus are in mutual reception and only ruled by one another, and also Neptune is ruled by Saturn ultimately. So it's Pluto, Uranus and Saturn