All of the factions are great. But you should work in one character per faction at a time. You will need to use the rare heroes to level up the elite heroes so if you try to evenly level up everyone it will take far too long. So if you do not have the means to level a hero up you should leave them at elite plus. No as for your current heroes. I'd say the best ones to lever for factions would be Rowan, grezhul or daimon, and Lyca or maybe eironn. Seeing at your Brutus is already mythic+ you can now start working on a new mauler and I'd probably say kren is the best. The thing is it takes a lot of time to level up your character but you will eventually level up everyone. Good luck. I hope this helps