From what I've heard the Carefresh mix is good. Unless I remember wrong, I think you mentioned going to Petsmart or something, so I assume your from the US. You can also feed Hazel Hamster (it's the US version of Harry Hamster which a lot of members feed). I've been giving my hamsters a mix from Sun Seed (only because it's so low corn content and is easy for me to pick out the high sugar bits for my dwarfs and doesn't have added sugars). As Roxie said you only need to remove the corn if you have a diabetes prone breed which is usually the dwarf breeds
Mealworms are a huge hit with my dwarfs, my syrian could care less about them tbh, and some hamsters are like that, but from what I've seen the majority love them so I'd give them a go!