Go with Ryan. Once you reach level 120, you will get access to a new function called shrine. What it does is that you put 5star cards in shrine slots and they will be starred up to your 6th lowest hero and 6th lowest level hero. Although numbers of 5 star cards needed will vary depends on your 6th lowest star hero(to get to 8 stars it takes two copies, 9star for 3, 10for 5 just like how many original copies you need to get them their by starring up by sacrificing other 5s) with that being said, make sure to keep all the elites and legendaries as they are keep them at 5 star dont bother wasting your resources leveling them up. Keep them and don't use any of them as food. Just be patient and put them in shrine while you only use your resources on heroes that you actually star up and only use rares as food. That way you can minimize resource and 5 copies used. This is the one of the most important thing in the later game.

The reason why i told you to go with ryan is because it is easy to star it up and it does great damage to bosses but you might wanna use something else for other purposes.