I only ever use a Hero(In 4vs4), and play Random.(People sure hate me, lol) So likewise I have plenty of experience on it. I've used everything a ton of times, even before making this account. Most useless heroes by far are the Firelord and Goblins(Both the tinker and ogre rider).
First is an agility hero with no mana to use his expensive caster spells. He's similar to the Naga Sea Witch, but she's intellect, and can actually afford her caster spells. Lightning also costs less and does far more than the useless Lava Spawn pets. Her nuke beats out his pets(Or Soul Burn, either way she wins in damage dealt. Soul Burn is 360 damage at Rank 3. Lightning is 750.), and her arrow spell is WAY better. You could argue that the pets will do more damage if they live long enough to split, but they are so frail and fights move or end so fast that this rarely happens. Lightning is dependable, Lava Spawn "rarely" helps at all. Take your pick. Did I mention that Volcano hurts your allies? That's pretty damn lame for an ULTIMATE. Imagine if Starfall did that... Anyway every spell he has is a total joke, making him the game's most useless hero.(Even his walk fails) Tornado does no damage to the enemy, but it's still damn nice to throw in on pile ups of enemies for the mass Slow and constant free Cyclones it will do. It's way better than Volcano. Enemies hate it so much they treat it like it's Starfall and try to stop it ASAP usually. As one of my favorite heroes, I've Tornado'ed groups of enemies in large fights tons of times and they rarely just ignore it. If you asked me if I'd rather have Volcano on my Sea Witch instead of Tornado, I'd insult you.
The Tinker has the game's WORST hero AoE spell(Yes even worse than Volcano). It'll take some numbers to explain, but basically its 300 max damage at Rank 3. That's shockingly bad. I mean, compare that to ANY other AoE. F.Lightning on Sea Witch=750 damage. Blizzard=up to 250 a wave(or to up 150 a wave at Rank 1). So Rank 1 Blizzard actually hits the damage of Rank 3 Rockets in as little as 2 waves AT LEVEL ONE. Then there's plenty of other AoEs, like Swarm, which is 1k damage at Rank 3. Hell even at level ONE Swarm does up to 300 damage. So Swarm hits the same damage as Cluster Rockets at LEVEL ONE. Chain Lightning is 270 damage at Rank 1(873 at Rank 3), which is close. So you see now? That's only some examples, you can compare it to ANYTHING. It's just depressing how bad this hero is, because you want a cool robotic/cyborg like hero to be good, or at least not a joke. He has a useless AoE, useless pets, a useless passive, and a Mech form that adds no real stats.(Compare it to Avatar or Metamorph. The End.) He comes in a close second on "game's most useless hero", but luckily his pets are slightly more useful than the Firelord's, his AoE at least doesn't hit friendlies(Also has a 1sec stun), and his Mech form at least gives him a useful siege to harass expansions or bases with. It isn't much, but it lets him at least beat the Firelord. His Rocket/Pet spam isn't much, but it's definitely better than a Firelord's laughable presence on a battlefield. You want Hero target abilities? Roll a Sea Witch and grab Frost Arrows, much better than Soul Burn.(Lol at Soul Burn users on heroes) I.E Storm Bolt+Slow Arrows=Win. Firelord used to be "ok" but then they killed him by Nerfbatting his passive into an arrow ability. It utterly destroyed him. Now he can't afford to even use it.
The Alchemist is the next runner up. His Heal heals enemies in addition to allies, making it the games worst healing ability. You have to have his Frenzy ability up for him to matter at all in a fight, and his Acid Bomb is completely useless. It's like Soul Burn or Cluster Rockets, it just doesn't matter and doesn't help enough to be worth using. I already did all the numbers with Cluster, so I'm skipping the time here. Just know it's a joke. Luckily his ultimate doesn't utterly suck too, as you can instant-gib expensive units, for instant EXP and steal the gold used to buy it too. Sadly these don't make him much better than the others. Only real use he has is to melee in short bursts with Rage, haul ass away like a sissy when it wears off, and heal outside of combat(Why you don't just use Healing Wave or Coil/Light is beyond me) or towards the rear of battles to avoid hitting enemies with it. He beats out the others due to having a heal you can still use away from enemies(A Heal is a Heal), while being able to do somewhat acceptable damage with Frenzy(+75% at Rank 2, that isn't Soul Burn in terms of fail let's be honest.), and having an acceptable Ultimate. This is a hero that has a leg up but needed further work, where as the other 2 are a complete bust.
I know I didn't look at the "mana cost" of all these abilities, but really it doesn't matter. Even if you want to argue "efficiency" on "damage dealt/healed per point of mana spent", by trying to say, "Oh they do less damage, but they are probably still efficient to cast", no, they aren't. They still lose. I mean Swarm may cost 110 mana, but at 1k damage at Rank 3... I mean look. I'll do one argument for you(Cluster Rockets). Swarm is 1k \ 110=9 damage dealt per mana. Cluster is 300 \ 75=4. Forked Lightning is 750 \ 110 =6.8(7 if you round up) So you see, even then these heroes FAIL hard.
It's no surprise that they are all "neutral" heroes. Blizzard has made sure most of them suck as much as possible, nerfing them into the ground soon after their creation. If you want another runner up, I'd say it's a tie between the Ranger and the Beastmaster for slot 4. You could argue that her Silence would allow her to beat out the Beast Master with the anti caster pwnage, but his pets add tankage to an army too.(Rank 3 Bear with 1200 HP and Bash procs on an enemy caster hero is no joke) But in the end neither have the mana to pay for their expensive spells, and that's their biggest issue really. If Drain Life for example was 50 mana to cast instead of 75, she would be WAY better.(That's an extra 25 mana a cast to pay for something like Black Arrow shots) As it stands neither hero has the mana to put their spells to any real use. At least the Pit Lord and Panda have a damn passive. You could always buy mana pots, but a better solution is to not use the heroes at all. As it stands now, those 2(PL and Panda) and the Naga are the only decent neutral heroes. I use the Ranger a lot just because she looks cool... that's the only real reason.(I cry uselessly inside every time they don't have Caster units to Silence though) If you're going to use her, I highly recommend you max Black Arrow at 3 and 5, while grabbing Silence as a secondary to save and use against casters.(Orcs and Humans love their casters) But Life Drain is completely useless, and if you get this at all, you're a newb(As explained by previous posters). Health pots are a much better investment.
But the only real "Race" hero you can complain about in terms of suckage is the Lich. But I think he's a pretty good hero, if you've figured out how to not use him for your first like newbs do. He needs a bodyguard for enemies to target first(And most will do just that). He's pretty dependent on Death Coils/Unholy and his own Frost Armor, but he's honestly far from being as bad as the others I've mentioned. This is a hero where it pays to buy those mana pots. Let's be honest the heroes I've mentioned are far worse than any Altar bought hero. People sometimes say the cow is bad but he has the games best aura, an AoE with a long range that hits buildings for full damage, an AoE stun, and an Ultimate that is FREE. You can't seriously believe such people.