The absolute piority is your main team of three. And I must say that, until you do not have 5 star legendary guys (the purple rarity ones) of your class upgraded at least at their 4th star, the best choice you can pick as alpha beta and omega guys of your team are 5 star starting and common guys (upgraded to the 5th star): while it's true that guys of a higher rarity class have better stats of guys of lower rarity classes, it's also true that stats gets a lot better as you upgrade the star level. so, gray guys at 5 stars are better options than purple guys at 3 stars.
Outside the main team, the priority of the other 3 star guys it's up to you. Among the 3 star guys, the purple rarity guys are the strongest, and having them upgraded will be of great help with places of power and champion performances, BUT they cost *a lot* to be upgraded, and approaching them as a priority, espacially if you are low to mid-low level player, could be frustrating. On the other hand, gray green and gold rarity guys are weaker, but more easily upgraded, and they are the foundations of your harem income. You should find the balance that fits you best (and maybe give some kind of priority to the guys you like the most, too)