My family and I have played this game for years, between its original heyday and more recently thanks to GoG. In my years of experience, these are the heroes I highly suggest, if you can get them, in priority from top priority (#1) to least priority (#10). Some have spell school requirements, which I will note at the end their entry.
1) Torin the Chosen One. Four Words: WALKING I WIN BUTTON. This guy does everything except ranged attacks. He fights, he can cast spells and he has a lot of good perks to boot. Requires the Life magic spell Incarnation.
2) Allora the Elven Archer. BEST archery hero in the game. Useful against flying units like Great Drakes and (especially) Sky Drakes. Just make sure said opponents don't attack her or she might not survive.
3) Mortu the Black Knight/Roland the Paladin. These two are just about two sides of the same coin. The main difference is that Mortu requires you to have Death magic and Roland requires Life magic. You can have one or the other, not both.
4) Aerie the Illusionist (not to be confused with the character of the same first name from Baldur's Gate 2). One of the most powerful casters in the game.
5) Ravashack the Necromancer/Elana the Priestess. These two casters are like Mortu and Roland in requirements, requiring Death and Life magic, respectively.
6) Warrax the Chaos Warrior. Best melee/caster hybrid hero. He also always has Arcane Power and Constitution along three other random perks. Chances are his chaotic tendencies will work in your favor.
7) Sir Harold the Knight. A very solid warrior who always has the Noble perk, which will take some strain off your gold uptake. Also has Super Leadership, Super Legendary and Constitution before his random warrior perk. Need I say more?
8) Death Stryke the Swordsman. This guy is just a combat machine who will give you plenty of bang for your buck.
9) Mystic-X the Unknown. This guy is even more random than Warrax. Still worth it, especially if you just can't get Warrax.
10) Shin Bo the Ninja. This guy is inherently invisible, giving him an edge against opportunities that can't detect him.

And of course, here are five heroes to avoid like crotch rot.
1) Theria the Thief. WORST. HERO. PERIOD.
2) Brax the Dwarf. A waste of money, even in the beginning. Many high-tier units or powerful summons will make a fool out of him.
3) Rakir the Beastmaster. This guy sucks just about everything. He might help with exploration, but there are even better heroes, let alone units and one Nature spell that can accomplish that.
4) Aureus the Golden One. Just plain lackluster.
5) B'Shan the Dervish. Only useful in the beginning for his Noble perk. Otherwise, worst archer hero.

Sounds about right.