I actually like Dark Side the best: it's the most varied and consistently interesting, I think. The Legend has late game balance issues (the last boss fight is bizarrely easy after a long stretch of difficult areas with few reinforcements). Armored Princess/Crossworlds is my second favorite- it's well-balanced and everything it adds is fun. Warriors of the North is a mixed bag: on the one hand, the Viking units are cool, and the experience levels are a neat way to reward you for sticking with favored unit types. But, WotN then goes and reuses a bunch of maps, slightly edited, and again has weirdly irregular difficulty. It starts out easy, then gets really hard, then really easy, then really hard again, etc. Also, the experience mechanic is kind of a pain when you run out of a unit type that you've been using as a lynchpin of your army.