In my personal opinion these are the heroes you should be working on and only on from that list
Zem, Taurus, tashi, octo, lumos, Kong, seraphine, Kaneq, xanos, firehawk
Tanya, radulph, solomon, valan, bloodspear
Driller, Arcturus, Daphne, goblin squad, chrona
Slim, gale, Gus
Free Heroes from shops you should farm right away in order of importance:
Arena: 1.nilya, 2.xanos, 3.valan, 4.goblin squad 5. Enrique
Epic arena: 1.Garrick, 2.lumos, 3.sagar, 4. Seraphine, 5.Solomon, 6.daphne
Crucible of fire(opinions differ I'm a top 10 player on my server and this is my opinion): 1.tashi, 2.octo (because of awakening or zem would come first), 3.zem, 4.taurus, 5.kong/tholin, 6.radulph (Tashi, octo, zem and taurus are all crucial Heroes so try to work on all of them as much as possible with Kong and tholin following not to far behind and radulph should come later but don't sleep on him either.
Hall of legends: 1.malrath(get him and work on him fast) 2.Cara 3.tanya 4.gizmo 5.tareth 6.nightshade
Guild shop: Avior (best arena hero here hands down) 2.jasmine (crucial for crucible of fire) 3.mirielle 4.bloodspear 5.firehawk 6.driller 7.lars
I know there are other Heroes in the shops but they were left out due to lack of utility and should not be worked on until all these other Heroes are at least 6 or 7 stars.
Campaign mode I'd suggest 1.Osiris, 2.arachna, 3.flora, 4.leah, 5.boomer (actually better than he gets credit for but still not very good lol) 5.elric
Mainly just work on Osiris from here.
Hope this helps, enjoy
I could go for a Cheese Burger right about now.... Just sayin