on the "light" side:

well, if you plan to do a lot of casting (I love me some rapid fire high crit fireball action - with the right gear and a bit of luck, you can get your fireballs to do 150 damage on crits fairly often), make sure you get a water mage in there somewhere.  Irwan is great (mantle of oceans is critical to have if you plan to cast spells a lot in combat).  For the firemage... doesn't really matter; all with fireskill will work fine.

best growth potential for assassin:  magar has great bonuses and starts at level 1 so very easy to get (though there is a level 9 assassin that has a huge bonus to crit for bows - can't recall her name).  with a decent bow and the right perks, either of these two are great at taking out single targets like opposing champs or casters with bows.  just pump init and crit damage boost, and by level 15, they should be doing 150-200 damage on their first hit.  both are also tarth, so get the double attack skill.  I've done well over 300 damage with that at level 15, with just a 22 attack bow.

defender:  Naegach (ironeer blood means lots of hps and good spell resist) There's a level 9 defender (Arvand?) that comes with dragon shield and a decent sword.

sword: Arvyn (comes with good skills, good weapon, and also comes with warboar mount)

Janusk is great early on just for cash flow.

all the rest are basically filler for need IMO.

The easiest way to win the game: 

Play Empire and create a Mage Sov -> save before reaching 100 Fame -> select the Gallowman -> Steal Spirit him for world domination -> watch end credits. Reload if you cannot pick him.

Apart from him, all quest heroes are fun and quite powerful. Heroes with special mounts or who can summon powerful units are also much appreciated. Praelius, with his equipment and summoning ability, is a great asset.

Honorable mentions:

- Bacco! He doesn't charge upkeep and has move 3. What's not to like?

- Ruah, lvl 1 Wraith with Loremaster I. If you make her a Commander, she can advance to Loremaster II and III at levels 3 and 4. Very nice.