It depends what you mean by stronger. Are you talking about flavour or caffeine level?
Dark roast coffee has a much stronger, more intense flavour than light roast. There's a reason it's called dark roast – the beans are roasted for longer, which deepens and darkens their flavour.
The beans themselves can actually be physically darker, too – light roasting will turn them light brown or even yellowish in colour, whereas dark roasting turns them black. Some aficionados think there's also a distinctive smoky flavour to dark-roast coffee that you don't get with light roast.
But as far as caffeine goes, light and dark have roughly an equal amount per cup – about one-and-a-half teaspoons of ground coffee per 5 fluid ounces (or 1/2 cup). There is an exception to this rule, however. Some light roasts are deliberately lightened up with a lighter roast to bring out other flavours. These light-bodied light roasts have lower levels of caffeine than the darker light roasts because they're made with beans that were already dark when roasted.
So if you want your coffee to be super strong, should you choose light or dark roast? If you like the flavour of dark roast but not the strength, switch to light-bodied dark roast; for example Starbucks' French Roast has more body than their Dark Italian Roast (darker).
For maximum strength (caffeine), go light on light. The darkest light roast is probably Starbucks' Blonde Espresso Roast which packs almost as much punch as their dark roast.