One of my best leveling potions is the combination of Blue Mountain Flower, Glowing Mushrooms, and Nightshade. This will produce a potion with three different effects, Damage Magicka Regen which is both valuable and great for exp, Fortify Health and Fortify Destruction come along with it to produce extra value and exp. As a potion combination its pretty useless for game play, but great for leveling and making coin.