Step drill bit - GIF

[KustomsbyKent]

I love step drill bits! As Marv said, these are what's preferred for making truly round holes in thin metal. They also make nice round holes in thicker material if you have step drills with longer steps.... I've got one that is for 1/4" thick material, up to 1/2" in diameter. I also have others that are the more typical 1/8" steps, and go up to 3/4 and one that's up to 1-3/8" diameter. I have a few in metric as well.
For drilling aluminum, if you have a lot of holes to do, lubricate the bit with WD-40 frequently, as that keeps the aluminum from galling up and keeps the chips moving.
I have had pretty good success with ones from Harbor Freight as long as you don't abuse them. They are easily half the price of name brand ones like Irwin, etc.