Personally, I avoid any sort of feed that has molasses or any sort of grain in it. All that is is sugar and starch that isn't healthy for the horse. It can make them more prone to founder, insulin resistance, colic, and other problems that start in the intestines.
As others have said, alfalfa pellets (yes, they are sold like a regular bag of feed, at my feed store it's about $12 for a 50lb sack). I also add a bit of oil, either canola or pure vegetable, not corn, and if you wish, flax seed. Rice bran is another good product for adding calories without adding starch/sugar. If you need to add a ration balancer, you can, but I've always had good luck with free access loose minerals.