I thought you meant what kind of cake
. (The answer to that, for me at least, depends on the recipient: chocolate [Duncan Hines Swiss Chocolate mix, with canned frosting] for my mother, pound [Betty Crocker mix, in a Bundt mold] for my father, or either spice [Duncan Hines spice mix, or a spiced up Duncan Hines yellow mix, either way with a maple or maple-cinnamon buttercream] or my own strawberry marble [based on a Duncan Hines white mix, with strawberry jam buttercream] for myself.)
As to design, well, I usually use whatever is the default design for the type of cake (piped strawberries, for strawberry marble, no frosting at all for a pound cake, or the family leaf pattern for anything else), with a blank panel for whatever I'm adding (in the case of my own birthday, just the number).
See my "Baked Goods" set for what I'm talking about. They're all simple designs, easily executed, with plenty of room for candles.
Of course, "frosted and served in the pan" single-layer sheet cakes are rather informal, but I don't stand on ceremony, and I haven't yet figured out how to tweak a strawberry jam buttercream so that it isn't sticky, so the pan provides enough protection that I can just cover it with foil, instead of putting it under a cloche, dome, bell-jar, or cake-safe.