I didn't list oatmeal (it would come in around 7th!), but I like it.
I like steel-cut oatmeal, which has more texture to it and doesn't get mushy (and somehow tastes a bit nuttier). I cook up about 4 servings at once (follow the package directions to cook for around 25-30 minutes), then put some of them in a bowl for later, and put the bowls in the fridge.
With the "just-made" stuff, I add soy milk, real maple syrup (not that "maple-flavored" crap), maybe some blueberries, and a tablespoon or so of ground flax.
The next day, I get one of the bowls of the already-made stuff (which is now just one big mass!), chop it up with a spoon, add soy milk, nuke it, then add my other ingredients.
Lentils only take about 15-20 minutes to cook, but I buy canned lentil soup.