If you had to buy a hero, the most expensive heroes are Doctor Strange, Iron Man, Rogue, and Spider-Man at 600 eternity splinters each. So, buying Rogue would get the most for your free token.
I bought Doctor Strange with mine because his play style is very different from my other heroes. Besides, he does magic, which looks cool.
Among female heroes, I have played Squirrel Girl, Scarlet Witch, Invisible Woman, and Black Widow to level 60 (acquired by random hero tokens from Adam Warlock). Squirrel Girl is ridiculously overpowered... which is her thing. I like Scarlet Witch and Invisible Woman... they both have great ranged/area powers with cool effects. But Black Widow wasn't my cup of tea... she's just a stealthy spy who sneaks around and shoots guns--not my idea of a superhero.