This is my personal list of heroes ranked worst to best. Some of them may be very, very tightly close to each other, so really it's a matter of how much you value the traits of these heroes over other heroes' traits.
9. Tramin
Definitely my least favorite hero. His skills are really boring and don't leave an impact. Liquid Nitrogen would be helpful but it cancels every time he moves. Flashbang isn't very useful as it takes quite a while to recharge for what it is. His boombot bombs are rather weak as well, as they move slowly and have such small AOE that they may as well have none. His base attack is really weak and he makes for a poor blocker. I recommend not ever touching this hero.
8. Veruk
I like Veruk, but he has to be here because if his strength, or lack thereof. His hero spell is insanely powerful in the early parts of the game, only to be killed quickly in the later stages. Veruk himself gets killed easily, as his regen doesn't trigger until about 20% HP remaining. Jigou can regenerate at any time, and heals by about 15 times as much health per second. Duelist therefore isn't very good as he rarely fights an enemy for long enough to benefit from this. His Goblin summons are also pretty fragile. I know, what do you expect from Goblins? It's telling that the two weakest heroes are both from the Orc/Goblin tribe. Maybe I’m racist against the greenfolk.
7. Mortemis
I really like the idea of making a zombie army. He has a good setup: spawn armies of units to block other enemies, and then cast his poison AOE to kill all of the enemies being blocked together. However, he has one major flaw: he has absolutely no range in his abilities. So, zombies rarely ever spawn, and he rarely ever gets to cast his abilities. You have to force them to trigger, which seems more like you're trying to make up for a weakness rather than capitalize on a strength. This makes him rather weak so I cannot recommend Mortemis to anybody.
6. Margosa
By default of her DPS, Margosa is here. I would rather use Asra to deal with high-priority, tough targets, but Margosa is admittedly more consistent as she can take out units on her own more frequently. Her hero spell is almost always active, which also means that she rarely ever uses her other abilites. If a player is like me and calls every wave as soon as they see it, her hero spell will also be charged, so Margosa may never leave it over the course it a level. Entering Beast Form resets her other skills' cooldowns, so it has a few downsides. She can also die in it, which happens against Frost Giants and most ranged enemies grouping together. However, if you don't mind a hero who just attacks and attacks in melee for an entire level, I can only recommend her.
5. Oloch
As cool as he is, it's sad to see that he surpasses a couple of paid heroes. First, he has the best dialogue in the game, as well as the best voice. Second, his hero spell is so fast that I often times don't even know where to use it, as he's powerful enough on his own. Third, as I just said, he is quite powerful, as his abilities deal really high damage and recharge pretty quickly. All of these attributes make him a fun and reliable hero to use.
4. Asra
Asra is so, so good. Some other heroes have an Innate Skill slot dedicated to being slightly faster, like Tramin's rocket jump or Margosa's bat. Screw those. Asra can teleport. She also has possibly the fastest teleport in the series, though Magnus Spellbane and Vez'nan were pretty fast, too. Regardless, her passive trait to destroy 6% armor per attack is so good. It doesn't take long for an enemy to have no armor at all, allowing the player to not have to rely on mages for high damage, but rather can spam archers and artillery, allowing their strategy to be all the more focused because an entire possible weakness in the player's defense has now been eliminated. Obviously Asra doesn't make all armor nonexistent, but she doesna good job countering it nonetheless. Her instakill poison move is also really good, as she gets 1 regular melee attack before using it, making it easy for the player to not waste the skill by accident. Her shield makes it easier to sneak in and use this ability, but it can also trigger when she is in a ranged battle with Elves or Gryphon Bombers, which always looks really epic. Her hero spell is also really nice, though I believe it resets her instakill poison which should be fixed in a patch.
3. Beresad
You can expect the 3 most expensive heroes to dominate the top of this list. Yes, he is the most powerful, yes, he has an instant-win button for a Hero Spell, and yes, he has the full gamut of an instakill, AOE, and anti-fast enemy skills. However, I just find Beresad boring. Unlike Bonehart where it was fun to try to get enemies in a group and kill just 1 so that it triggered an explosion, or Phoenix in which you tried to get her killed despite being a flying unit as much as possible, Beresad doesn't really have a playstyle. When you know one ability is ready to be used, you use it and that's it. As powerful as he is, I can't recommend him.
2. Doom Tank
Although he cannot be blocked, his expendables can, so you basically get 3 units to stall rather than the typical 1 from this hero. His AOE slam is massive and can even stun flying enemies, making him really good if you place him directly in the middle of a huge battlezone, something that normally got heroes killed in the other games. His fire spin is actually worse than his normal attack because it deals its total damage over a long time, but that's not a problem unique to this hero so I don't count it much against him. He's just really solid and reliable. Highly recommended.
1. Jigou
The reason I have ranked him the highest is that he is the most consistent and reliable hero. Unlike Beresad, he can actually block enemies, and is almost totally invincible. 900 health, 24% armor, 30 HP per second, and a 9 seconds of invulnerability ability make him able to block even the final boss for over 15 seconds! His ranged attack is very fast and can be used to make him slightly better against tougher enemies but he's no DPS machine. However the support he gives to towers is immense, so it really doesn't matter that is damage isn't as insane as past heroes like Oni or Dante. His AOE stomp is really, really good, and his hero spell, while it has inconsistent damage sometimes, is also powerful. I recommend him the most because he basically amps up the traits of a normal hero to 11, rather than have some sort of unique feature like being flying or unblockable like Beresad and Doom Tank as an "excuse" to be maximum price.