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Metals are good conductors as the electrons in their outer shell are loose and can plunge out of the atom with the application of the slightest force (voltage).
Silver is the best metallic conductor, then comes copper.
Why is one metal better at conduction than the other one?
What determines the difference in conduction between 2 metals?
Why is silver a better conductor than copper, and why is copper a better conductor than iron if all of them have loose electrons?
This is, ultimately, a question on solid state physics rather than chemistry. Further, the OP indicates that they are in high school, which kind of limits the depth of the answer that might be useful to them. However, I will try to make a simple, yet detailed answer.
As atoms are brought closer and closer together, their electron clouds overlap and interact. When a crystalline solid is formed, one abandons consideration of the atomic energy levels. Instead, one seeks out the Bloch functions to describe the electonic states - the Bloch functions explicitly consider the symmetries of the crystal. Further, they represent extended electron states, those that extend across the entire crystal. They represent the allowed (energy and momentum) combinations of electrons.
For a metal, the highest Bloch states occur as a band of states which is only partially filled. Unoccupied states lie just above states with electrons. To move an electron about, it needs just a little bit of energy, and off it can go. (Fully filled bands do not allow conduction - for any electron going one way there is one going the other way. This needs a good grounding in solid state physics to fully grasp).
OK, in general then the conductivity should just be the number of electrons near the line between occupied and un-occupied. Kind of. This line is called the Fermi surface, and things get more complicated quickly. The shape and connectivity of the Fermi surface impact conduction dramatically. Lets take a deeper look at copper and iron.
Copper is a simple noble metal with a beautiful near-ideal Fermi surface (see Ashcroft and Mermin's Solid State Physics book, or C.Y. Fong et al., Comparison of band structures and charge distributions of copper and silver). It turns out that the Fermi surface of copper is almost exactly what one would expect from the free electron sphere. Electrons can move easily in any direction, the surface is fully connected, life is good.
Iron is much more complicated. Following along from J. Callaway and C.S. Wang, Energy bands in ferromagnetic iron one finds two major differences. First, because iron atoms indeed have individual magnetic moments, the overall band structure is split into two different spin states for the electrons. So, each spin state is separated from the others, reducing how each can move (relative to copper, were such splitting does not exist). Second, and more importantly, the Fermi surface looks nothing like a free electron sphere. Instead, for iron the Fermi surface is not fully connected, comprising instead of multiple 'pockets' of electrons that do not communicate directly with the other pockets (and each spin state has a different set of pockets). To move electrons around, they have to 'jump' from one pocket to another through scattering. This makes it much harder to move the 'free' electrons in Fe around to get net conductivity.
An analogy would be the difference of crossing a stream using either (1) a nice smooth bridge (copper) vs (2) jumping from rock to rock (iron). The band structure for copper just makes it so much easier, so the resistivity is much lower.
TL;DR.
To get the "real" answer, you have to have a good understanding of the underlying quantum mechanics. I'm pretty sure you're not ready for that, yet! (and note that this topic would require several semesters of quantum mechanics, it's in the sub-discipline of solid-state physics...) So, my answer will have to be simplified.
You're taught in school (perhaps) that there are 3 types of chemical bonds: ionic, covalent, and metallic. The world isn't that simple in reality, while all chemical bonding is electromagnetic, there is no "perfectly pure" ionic, covalent or metallic bond. If you accept the idea that metallic bonding involves all of the valence electrons of the atoms in the crystal (metals generally exist in small crystals called grains or crystallites) then we can simply say that Ag, Cu, and Au all have a single valence electron (per atom) and it turns out that this electron is fairly well shielded by each atoms' inner electrons from the nuclear charge which makes it much easier for these electrons to move, compared to, say iron. One way to look at it, I suppose, is that the road is less sticky (I wonder if calling it smoother would be a better analogy?) so that these puppies can move more easily than the valence electrons of iron. So, the answer is that there are degrees of "looseness". There are also grain defects and grain boundaries to cross. And also you should always keep in mind that electrical conductivity is temperature dependent. Stating categorically that silver is "the best" electrical conductor is more fan-boy enthusiasm than objective fact. The "best" electrical conduction is with superconductors - but of course then you need to consider the amount of current they can carry before they fail so "best" takes on a magnetic field dependence..."best" isn't a very scientific word is all I'm saying...there's almost always too many qualifications involved.
If you want to get a more correct answer, go to wikipedia's articles on Nearly free electron model, Metal, Fermi surface, Electrical resistivity and conductivity but I'm afraid that they're pretty hard to understand even for someone who has had some quantum mechanics.
I personally like my analogy that the road for silver's electrons isn't as sticky, but that's not to be taken too far... You actually have to consider not only the 'resistance' due to the nuclear (attractive) charges as the electron zooms by, but also the path the electron follows (the crystal lattice, how zig-zag the road is) and the defects and boundaries is must pass through (stop and pay a toll).
