In most cases, rarer heroes are better than more common ones. The rarities are: Common, Uncommon, Rare, Very Rare and Super Rare. IMPORTANT! The rarity is different from the number of stars. While usually the rarer heroes have a potential of more stars, you can still have a more common one that could reach the same amount of stars. For example, Smasher is a very rare hero which can get to 6 stars, even though most Very Rares are capped at 5. So just looking at the number of stars could be a bit misleading. And this would be the most important rule to have in mind: rarer heroes are better than more common ones: they have better starting stats and also better skills in most cases.