You can take a hero with Logistics. If it's important to explore more of the map early on, and in the case of the Elves, it's almost mandatory because Dendroids are powerful (I think you understand which hero I'm talking about).
If you're playing as the Necromancers, you definitely need Aislin, and not just because of "Meteor Shower." Aislin is given skills that are perfect for a mage.
For the Barbarians, everything is also obvious (Crag Hack, of course!).
If you're playing as the Elementals, I forgot the name, but the point is that magic should be a priority, and fire magic in particular.
For the Swamp, it's Stig... because Stig + Top Gear is a great combination. But seriously, there just aren't any other decent heroes for the Swamp; it's impossible to level them up, and they get terrible skills at each level.
For the Stronghold, I think everyone has a choice between Deemer and... that shaggy guy with "Resurrection." Although, again, if speed is important, Gunnar is very good (if you don't choose him right away, you have to try hard to buy him).
For the Demons... I can't say anything about them. I played a couple of campaigns with them and a couple of 200-tile maps out of principle, but that was a long time ago, and I don't like their Castle.
Ah, I almost forgot about the Humans. They are the only race for which water magic is important because the prayer on the human army is powerful (and the choice is based on this).
That's my opinion, if anyone cares.