My fiance loves it when I make "butterfinger cake". Chocolate cake mix + 1/2 cup peanutbutter. And then a batch of milk chocolate frosting with crushed butterfinger candy bars mixed in.
It tastes just like a big, moist, delicious butterfinger.
But MY favorite cake is "Jello cake". White sheet cake baked up, and then you poke a bazillion holes in the top with toothpicks/chopsticks, and then pour fresh jello on top (still liquid, not set up). You can use any flavor, but my favorite is blue raspberry (which is getting harder and harder to find!). The liquid jello seeps into the holes and makes a really pretty pattern when you cut the cake, and that makes it very moist. Put it in the fridge to cool for a few hours, and then top with cool whip!
I can eat a whole cake of this by myself in just a day or two.