Heroes (not the entire spec, just the contribution made by the hero card on its own):
One of the key values a Hero provides is giving you a way to spend gold to impact the board, without using any cards. For stand-alone Hero power, I will assume that you have not teched any of that hero’s spells, and have no plans to in the future. Any “survivability” value will be reflected in grading the individual spells, where having a flimsy hero will impair the grades for that hero’s spells.
A – Superior. Has a significant effect on the game. Stats are at or above average. Impactful at all stages of the game. All abilities and stats are efficiently costed. Special mention for having an immediate impact on the board the turn they are summoned. Examples: Grave, Zane
B – Above Average. Has efficient stats, and some immediate impact, but perhaps only when raised to max level. Impactful in the early stages of the game, and doesn’t lose too much value going late, or vice versa. Examples: Onimaru, Garth
C – Average. Gives you reasonable stats, at a reasonable cost (baseline is being a 2/3 at level 1 and a 3/4 at level 5). Has a minor immediate impact when played or raised to max-band. Has a strong impact on the board in the early or late game, but not both. Examples: Setsuki, Geiger
D – Below Average. Too expensive for the stats provided. Abilities are niche or not impactful. No immediate impact. Examples: Jaina, Prynn
F – Unplayable. Even if you have this Hero available, you would never summon it, because the Level 1 body isn’t worth giving your opponent 2 free gold, and it isn’t worth the gold to level him up. Example (and sole hero to get this grade): Troq
? – Build around. Not worth it for the stats alone, but with the right supporting cast of units, could be an A or B quality card. Note that this still assumes you are not teching any spells for the hero. Examples: Quince, River
River Montoya – ? River has average stats, and low cost, and can serve as a fine filler play. Her abilities are lackluster, as sidelining is only rarely useful, and her max-band doesn’t provide significant payoff in most situations. However, in a hyper aggressive low-tech build her powers can provide significant bonuses, helping you compete with opponents who are teching up, and let you force through base damage. Also part of a combo with Smoker and Drill Sergeant / Blooming Ancient to generate unlimited +1/+1 runes, or the less pie-in-the-sky combo of Garrison + playing tech 0 units for less.
Troq Bashar – F Troq has average stats, with the option to “go big.” Unfortunately, he has no significant abilities, and the 5 & 8 level-up curve isn’t worth the gold without some additional benefit.
Zane – A. Zane is smaller than most other heros until his max-band, but his immediate impact is unmatched. Being able to add a hasty 2/2 or 4/4 (plus effectively 2 direct damage if you push away a Squad Leader) to the board is something your opponent has to keep in mind the whole game, and even then he will give you a reasonable rate for card-free damage. That’s not even mentioning his mid-band that usually gives you 2 gold and/or cards before Zane dies.
Drakk – A-. Drakk has awful stats, but some of the best abilities in the game. +1 ATK to all your attackers will usually let you break through any patrol zone, and giving any unit Haste is crushing.
Jaina – D. Jaina has weak stats, and expensive leveling for those weak stats. The only hero who never gets above 3 hp leaves her vulnerable at all stages of the game. Her max-band is strong, but again more expensive than is usually worth it.
Midori – B+. Midori has standard large hero stats, but being able to fly (albeit only on offense) makes him a substantial threat that must be answered, especially when backed by the Green or Neutral starter and a potential +1/+1 rune. In addition, his mid-band makes it easier to defend him – promoting tech 0 units to 2 cost 3/3s, and making tokens of all stripes just a little bit better.
Calamadra – A-. There is no better early game hero, combining the best stat line in the game with the ability to apply those stats at will, and a built-in way to recover from card and tech disadvantage with Tigers. Late game, however, her board impact is limited, and her abilities are lackluster.
Argagarg – C+. A poor combatant in himself, maxing Arg still contributes 4 attack and 9 defense split over 3 bodies, one of which has anti-air, for the bargain price of 6 gold. This is a reasonable card-free investment that can compete with the best of Tech I, but is outclassed once Tech II cards start arriving. A fine use of gold, but not anything your opponent will have to play around.
Vandy – A. Vandy is tied with Calamadra for best raw stats. Like Cal, she can make up for a card loss with her exhaust ability, and her inherent resistance makes it just a little bit harder to get her off the board. Her ultimate is easily controlled, and often gets a significant attack through immediately. However, her late game potential is limited to being an efficient body. Fetching Dark Pacts, Soul Stones, and even sometimes Metamorphosis gives Vandy continued relevance into the late game.
Garth – B. Garth has below average stats at any cost level, but makes up for it by generating extra Skeletal blockers at an affordable rate. He also lets you regulate your Tech II access by guaranteeing you access to the Tech II unit of your choice as soon as you start a turn with a completed Tech II (and bringing along a 3/4 hero as a bonus).
Orpal – C+ Orpal is slightly smaller than most heroes in raw stats, but dealing damage as -1/-1 runes makes up for it by helping your other attackers survive. He also provides a moderate amount of immediate board impact if supported by disposable units and leveled up, but the overall package is just ok, not anything special.
Bigsby – C. Bigsby brings average small end stats, and generally applicable mid and max band abilities. He gets bonus points for helping regulate your draws – specifically, you can bring in two Tech II cards on turn 3, planning to build your Tech II on turn 4, and if you happen to draw one of them on turn 4, play Bigsby to reserve it.
Onimaru – B. Onimaru has the largest stats of any hero – the +1 ATK means he wins fights against any similarly leveled hero. He does have the 5 & 8 leveling costs, which diminishes his ability to heal, but he can still reach the critical 4 attack threshold at level 5. Combined with a very strong max-band that is worth triggering from ahead or behind, and you have a very solid choice. Fun note: the Peace strategy is building destruction, and the Peace hero is the only one who can destroy a tech building by himself.
Quince – ?. Quince levels quickly and builds up a lot of HP, but like Arg he doesn’t have much board impact from himself. Unlike Arg, Quince can single-handedly dominate the game, given the right units on your side (or your opponents). Quince particularly likes units with Haste, or with high attack/low hp, since the Mirrors are inherently fragile.
Prynn – D. Prynn has horrible stats (only slightly better than Drakk or Garth), but also lacks any sort of useful abilities at start or mid-band. She is saved from an F grade by having a hugely impactful max band ability, making her an excellent late-game spell.
Geiger – C. Geiger is incredibly average. He’s got basic small-range combat stats, a max-band that has moderate and affordable impact, and scales through the late game, remaining moderate throughout.
Vir – C-. Draw manipulation that is slightly worse than Bigby, with slightly worse stats than him. At the top end, you get an extra HP over the average, and a large (if delayed) body. Overall not a hero you want to be playing early, but works well in the midgame to provide some support heading into the late game. This narrow window of viability earns Vir his below average grade.
Grave – A. Grave reaches the 3/4 level cheaper than most, and has Readiness plus Sparkshot in an affordable body. Then he comes with an option to pay 4 gold and exhaust to buy a Doom Grasp, which is the best spell to have on-demand. Late game, he can be a proactive answer to some Tech III options, and reaches 4/5 for less than most heroes who reach that stat line.
Setsuki – C. Defensively, Setsuki does little to protect your other cards and buildings, with her below average stats. Offensively, Setsuki is again under-statted, and swift strike only matters if you can kill something with it (which you can often do, but only in conjunction with another attack). This makes her roughly equivalent to an Arg mid-band that has more +ATK and no +Armor. Her max band, assuming you can play the cards you draw, is one of the most potent in the game, but if your tech buildings are knocked out, spamming Tech 0 units is not going to get the job done.
Rook – B+. Rook has the best Level 1 stats in the game, and the most resilience of any hero (albeit at a high cost). Played early, he’s a brick wall. Played late with immediate max-band he’s a stone wall (barring specific anti-hero spells from the opponent). Rook is a one-dimensional hero, but he’s the best at what he does.
Reminder before you go posting comments: these ratings assume that you are not, and will not, tech any of the hero’s spells (presumably because you are using your tech slots for something else).