Which purple heroes are best?

For combat all the purple heroes are generally the same. You will need to use a few of them at the start but don't worry too much about investing in them as you will hopefully soon start recruiting orange heroes. These are much better and over time you should recruit enough to replace all the purple heroes in your legions.

Wen using them (or any hero) pay attention to the skill you unlock, for a few heroes some of the skill only work when they are leading certain troop types. Also some of them have a 'range'. the smaller the range the close to the front they need to be (ie placed in the front row)