Ancho is mild (it is the dried version of poblano chiles). The guajillo can range from (usually) not-quite-mild to the low end of medium, most often in the middle of that, I find. Pasillas are the dried versions of chilaca chilies. They are mild but flavorful and used especially in moles. Negros can be pasillas, a type of pasilla, or another type of chile, depending on where they are from.
New Mexico chiles range from mild to very hot, the majority ranging from mild to the low end of medium.
Mix a little softened butter with some oil (I use unsalted butter and olive but use whatever is at hand) about a 50-50 mix, totaling roughly 2 tablespoons. Divide the mix into four (just separate onto large soup or stirring spoons) then sprinkle well with each each of the chilies, powdered, separately, except the chile de árbol, mix in with a smaller spoon (wiping the spoon well before mixing each or using a separate mixing spoon), and allow to sit a few minutes while you stick a slice of plain bread in the toaster.
When the bread is lightly toasted immediately remove it, cut into quarters, then immediately spoon the mixture onto each piece and sprinkle lightly with salt. (You want the heat from the bread to volatize the chilies' flavors.)
Taste the pasilla, then negro, then guajillo, then NM. If you wish rinse your mouth with club soda or seltzer in between -- or you can do the tastings more separated by time, prepping the butter-oil mix in smaller quantities and simply toasting a small piece of bread for each tasting.
You should get a sense of the flavors this way. The pasilla and probably the negro should be deeper, with deeper fruit notes and a kind of richness (ancho is similar but lighter in flavor yet still rich); the guajillo should be kind of tangy, with tart berry notes, but with a warmth of ripe tomato; the NM is dependent on strain/variety and ripeness but will likely be lighter in flavor. Heat can vary, as noted.
The chile de árbol is not necessary to taste this way but you can. It should have good heat, similar to cayenne, but with better flavor.
Proportions in rubs depends on rub type and what you are using it on. For deeper chile flavor go with the deeper flavored chilies with, perhaps, some guajillo for a little zing. Add the c de a for heat after mixing the other chilies. This sort of rub is good for low/slow. For something grilled quickly -- steak, say -- I like something a bit brighter from the get-go since the caramelizetion of the steak's surface will automatically add depth. For something like this I'll often go with all guajillo or mostly guajillo with just a touch of, say, pasilla and c de a added.
Play with the proportions. Don't make too much rub at once so you can vary amounts and/or leave things out. You'll discover what you like and in what applications as you go along.
