Yony, Yes, I got that you were trying to pose a simple question. But there are no simple answers in the way you posed your question.
If you're just learning astrology, see if you can follow what I'm saying. If not, I am happy to clarify or elaborate. Let's start with modern astrology, the kind most people practice on this forum.
We're agreed on 12 signs. Beyond that you can read pop-schlock astrology sites about signs as baskets of static personality traits, and pick and choose which traits you like and dislike. Which turns your astrology into a highly subjective ego-centric activity. Obviously, Astrology For Adults gets way beyond this.
Astrology works with horoscopes for real people and events, not disembodied abstractions. Thinking of signs as disembodied abstractions may be an interesting activity, but astrology doesn't work that way.
In a horoscope some of those 12 signs will have planets in them, some will not. A sign's "strength" is only relative, depending upon (a) whether or not it has any planets in it, and if so, which ones. (b) Whether a sign is at a chart angle, notably the ascendant or MC. Throw in other non-planetary points if you wish, like the moon's nodes or the part of fortune; but basically look at planets.
If you do traditional western astrology, you would probably also consider (c) the sign's position on the house cusp. Angular strengthens it. The 12th, 6th, or 8th house weakens it. [Signs and houses are not the same thing, incidentally, except in medical astrology and in some modern astrology which normally I don't recommend.]
So signs are stronger or weaker by virtue of their contents, not intrinsically on their own. Put it this way: is your wallet more valuable if it has $1000 dollars in it and paid-off credit cards, or is it more valuable with $2 in it and maxed-out credit cards? One is "full" and the other is virtually "empty."
A sign has no intrinsic strength or weakness on its own, independently of its content and location in the chart.
You might think a fixed sign on its own is stronger than a mutable sign on its own, but astrology doesn't work this way. A sign tells you how or in what manner a planet operates, similar to an adverb or adjective in a sentence.
Suppose someone has a stellium in Pisces which includes the sun, Jupiter, and Venus; and Pisces is the sign on the AC (rising sign.) Leo, in contrast, is on the cusp of the 6th house, and has no planets, angles, or sensitive points in it.
Obviously Pisces is the strongest sign in this example, despite being the mutable water sign. It is angular. Jupiter is domiciled and Venus is exalted (traditional) in Pisces. Leo still acts like Leo in this person's 6th house of health, illness, work, and service, but it isn't a strong player. Moreover, because Leo's ruler, the sun, is in Pisces in the first house, that Pisces sun has a big say in how those 6th house matters function.
The formal way of saying whether or not a sign has any planets in it is calling it "tenanted" or "untenanted."
It actually works analogously to a landlord with 12 investment properties (signs.) Some of those properties are occupied by tenants who pay rent. Some of his properties are untenanted. The tenanted properties add to the landlord's financial strength. The untenanted properties do not. [Of course, the landlord might have bad tenants who trash his property and drag down his income, but that's another astrological story.]
So I hope you understand that signs are not inherently weak or strong on their own, in the modern astrology I and most people on this forum usually practice.
In Hellenistic astrology, if you want to get into that branch, signs are differentiated by virtue of the their planetary ruler, or planet that is domiciled in them. (See the beginning of Vettius Valens, Anthologies, for examples.) Saturn was the Greater Malefic, so this meant that its signs of Capricorn and Aquarius where pretty horrible. Not necessarily weak, mind you, just trending for really wretched people. Jupiter was the Greater Benefic, so this boosted its signs of Sagittarius and Pisces.
But even in this system, so much depends on the disposition of the entire horoscope.
Which sign is weakest or strongest?
It depends......