I think it’s very situational. Nalfar is a great hero in endurance games, like Cavern Survival and Sacred Statue, but terrible in Harrowland and Dueling Grounds where there’s less time to “bulk-up”.
My personal tier-list would be
S Tier
Usara: Surprisingly she does well in dueling grounds because she has the potential to overwhelm enemies(Especially warriors) with a swarm of units. Robobombs and Yetis can be commanded, which can help with reducing wasteful usage. Stat-swap can help you stay alive for longer, too.
Illia: Illia’s overall balanced stats, along with abilities that counter both rangers and wizards through projectile deflection and swarm control make her an amazing, good cost-performance hero. The only hero that can even potentially outduel Illia is Okar. She is basically immune to other heroes.
Senick: Senick is amazing despite being a little over 1,000 
High overall stats, allow Senick to tank many hits and deal large amounts of damage. He comes with a heal ability which can let him endure a few more extra seconds. The existence of speed ring and invisibility ring basically negates the disadvantages Senick has versus Ritic. While Ritic can probably beat Senick in a fair duel, the price difference is just too different.
A Tier
Ritic: Generally, Ritic is only good if you have a backstab weapon, which allows dealing hundreds of damage. Fragile, and can be beaten with a smart-enough Senick. If Ritic was a little less expensive to obtain, he would be in S-tier. The damage difference between Senick and Ritic is pretty negligible, so in most brawls, you’d use Senick instead of Ritic. Tornado unfortunately blocks your own projectiles, too.
Okar: Okar is simply the definition of a tank. (Man, he’s huge) Low speed can be compensated by the Ring of Speed and Okar’s long-range throw ability, as well as stomp. Because of his extremely high damage output, he’s amazing in a tight one-on-one combat, but is prone to being kited, and a well-played Illia can counter Okar if the map is large enough.
Naria of the Leaf: Naria is severely underrated because people seem to forget about her envenom ability, which greatly increased DPS output. She also has a built-in invisibility ability, allowing you to double-invis or use a different ring instead. From personal experience, she does pretty well in dueling grounds and I’ve once had a #1 strat for a few months.
B Tier
Omarn Brewstone: Before the rework Omarn would be in D-tier. Now he’s up here because he can heal, speed-up, increase damage, and throw Brewball-certified explosive potions! The ability also has a short cooldown, which means Omarn can adapt to certain situations using these potions. While he’s not necessarily good at a specific game mode, he’s versatile.
Nalfar Cryptor: I can already imagine anger fueling from some Nalfar fans, here… Yes, I agree Nalfar is unkillable in Cavern Survival and useful in Sacred Statue. Where else, though? He’s too situational that I would never use him elsewhere, as Omarn, Pender, Usara is easily a better option because of higher offensive values.
Zana Woodheart: She’s another Dueling Grounds favorite because of her arsenal of enemy-weakening abilities, and a built-in windstorm spell! While she’s not the best hero choice when it comes to many combat scenarios, she’s not only a great utility hero, but the fastest one. Even Usain Bolt can’t outpace her.
C Tier
Pender Spellbane: A few years ago, she would’ve easily been on A/S tier. She has unbelievable damage and speed, at the expense of health. A literal glass cannon. However, the introduction of Usara, who has a higher damage multiplier bumped Pender down. Pender is still great for speedrun challenges. (Check out Nick’s Speedrun Video)
Arryn Stonewall: Sure, she can throw painful razordiscs and increase knockback, but because of her overall low stats, especially her damage multiplier, heroes like Hattori would be a better damage-dealing option. Knockback isn’t really special and can be harmful if you knock back ranged enemies, so her warcry ability is mostly pointless. She does have speed, which may potentially save you from being bombarded by Thoktar’s fireballs.
Hattori Hanzo: High damage is very preferred, but Okar also has high damage, but also high HP, which Hattori heavily lacks. He also doesn’t have any cool abilities other than a speed-boost to him and his allies, so he’s down here in C-tier.
D Tier
Anya, Tharin, Ida, Alejandro: Free heroes are actually pretty good because they have a decent balance of health, damage, and speed. Don’t be fooled by Tharin’s description.
Armando Hoyos: He’s free, and comes with a charisma magnet which is a weaker, simpler version of Usara’s magnet-field ability. With cleave/flash, he’s actually very good.
Ned Fulmer: You can become invincible half of the time with starter hero stats, which is, for a hero that is free, extremely strong.
F Tier
Amara Arrowhead & Miss Hushbaum: They should be pretty self-explanatory.
Gordon the Stalwart: He’s basically a starter hero with slightly more HP, and slightly more speed, but way less damage and the ability to throw a hammer. (Adds ~16 DPS. Not very worth it)
You can also read a guide evaluating each element and giving each hero a grade here: