I keep upgrading all of my heroes, but I want to focus on the strongest most impactful Hereros. Is there any guide for this?
Yes! I believe a forumer named Emitz makes a chart
Emitz quit the game about a month ago. Musk (aka @Musketeer) makes a Tier List that they update pretty often: Hero tier list [Update 2.7]
Isn’t their chart still relatively accurate?
Emitz’ list? Sure, but since he’s not keeping up with the game anymore, his Tier List will eventually become outdated.
Unless Musk quits the game, their tier list will usually be more up-to-date.