I was working on a system to grade the heroes, but it got really complex. Here's a rough take on it (not personal preference, but my opinion of how overall good a hero is)
Tier One: the Leaders
Legacy: He can shut down the villains deck, damage, heal, boost. He can deal really good one-shot damage when needed.
TLT (the only promo I'm including): Tachyon is really strong, Team Leader takes 90% of her strength and adds possibly the best support power in the game, as a base power.
Omnitron-X: what can't he do? He is involved in so many game breaking combos it is unreal. When set up he can draw, play cards, deal damage and heal allies and then skip his turn.
Wraith: jack of all trades, master of. . . all? Her only downside is she needs equipment, she counters that with the single best pulling equipment card in the game and lots of copies of her goodies.
Tier Two: The Juggernauts
Haka: he augments a dizzying array of damage with the Haka line of hand manipulation + a bonus cards. (seriously draw 2 discard 1 is a great card in most decks, he gets a bonus added in as well) Savage Mana and Ground Pound are both epic additions that turn a really bruising dealer into a juggernaut of enemy destruction.
Tempest: Massive one-shot damage, great array of powers, damage bonuses and start of turn damage, ongoing and environment destruction, he is an engine of win. His only downside is he can be really frail sometimes. Take away the +1 damage taken on his one card and he's tier one.
The Scholar: Amazing healing, solid damage, somewhat hard to maintain. DDA is one of the top cards in the game, and scholar burns his deck. his deck mechanics are top notch, maybe only outpaced by the next entry on this list.
Chrono Ranger: Superior deck mechanics, he can get knocked down and be back on his feet in a minute, and if you don't knock him down he will stay on the big bad guys tail gunning him down in record time.
Tier Three: Winning.
Tachyon: the only thing holding Tachyon back is how easily she runs out of cards when she is really rolling if she can't find trust fund. She needs help to dominate the field, but with that help she crushes everyone. Without it she still drastically improves your chance of winning.
Fanatic: No one burns down a villain like the angel of vengeance. Her damage control is amazing, but requires set up and maintenance, her damage output can b amazing, or just ordinary if things don't go your way. Aegis is the best heal in the game, if it is up when you would die, and End of days can be game breaking in the right setup. Like Tachyon sh'es a good fit in any situation, evil crushing demi-goddess in the right situation.
The Visionary: that help some heroes need to make it from alright to awesome, or really good to gods of battle? That help is visionary. her base power can help anyone's hand, and TtE can break the limitations of a number of heroes. She can also just shut down a villain or environment, and has a ton of shenanigans to pull with WtM. Her fallback of Cocoon is good, but finding ways to survive without it is even better.
The Argent Adept: Where the Visionary is about raising the floor of an ally, Adept blows off the ceiling for his team. He can help you draw a card if he is set up right, but his real strength is doing a ton of things at once. Early on he's good, when the team is set up he's the conductor bringing the whole team to new heights of success.
Tier Four: The Good
Ra: lots of damage, lots of it. Some really cool side effects, but mostly fire and lots of it. Need damage, Ra's your boy.
Absolute Zero: one of those heroes you build you team around, because otherwise he might not be that much help. Set up in the right team he's dominant, but his set up takes a bit and he may need help to shine, but dang, he's worth it.
Nightmist: She doesn't need help like AZ, she isn't one sided like Ra, but she can backfire pretty badly. Set up and rolling she is an unkillable destroyer, but minus a few key cards and she's just killing herself. Depending on your experience with her I could see moving her up.
Unity: She fills a very cool niche. She needs way less support than people tend to think, and Hasty Augmentation is amazing. Stealth Bot, Cryo Bot, Unity is really good.
Tier Five: The best for last.
Mr. Fixer: Many teams just don't need him, but many benefit a lot from him. know your enemy, know your allies, and you will be victorious.
Expatriette: she ranks down here because every once in a while you get that game where your deck hates you. Her damage is consistant and versatile. Her ammo is really a good mechanic, and has a lot of uses.
Bunker: maybe the most set up intensive hero. He has many weapons, but only a few great ways to use them. His best set ups still limit him compared to other heroes, but he can do many things, and with the right support and a little luck, he can dominate.
Real quick addition here:
I couldn't help but notice when I looked back over the list that I still managed to group them in like/dislike categories.
I do not enjoy playing tier 1 heroes, they are just too good without enough work required.
I don't really enjoy tier 2 either, although if I need a really powerful hero to play I go Tempest.
Tier 3 are my go to destroyers, when I want to be the team MVP I go for one of tier 3's heroes.
Tier 4 I don't go for either, they are good, but not my preference.
Tier 5 are my favorites, I love Fixer and Expat, they contribute so much without dominating, they are really strong at rounding out a team and filling in gaps. Bunker is relly fun when you have some one playing a support that works well with him, or when I want a bit more adversity.