And of course, these are all simply my opinions. If I'm neglecting to see some major strategic flaw that makes Dirael's wasp swarms game breaking or something, or if I'm forgetting someone starts with 20 black dragons, because I'd frankly be curious to know.
Stronghold:
Best-- Haggash. No question, for me. Early game is one of the most important stages of any game, and she simply has the best start for orcs. She starts with the most centaurs and what I consider the best starting skills.
- -Memory of Our Blood makes it so 1-3 attacks will boost Centaurs to rage level one in any fight.
-Attack is self explanatory. Simply a good skill to have when your only real goal is killing enemies before they have an opportunity to attack.
-May take Archery to boost her pet Centaurs at first level.
-Memory of Our Blood gives her second fastest path to Battle Elation, and fastest path to Retribution, able to have both and Archery by level 10. (Not likely to be available, but wouldn't that just be amazing?)
Honorable mention for Urghat because starting with Pathfinder is flipping sweet.
Worst-- Kilghan. His bonus gives you more goblins and keeps them from betraying you. It takes a long time for the weekly goblin joinings to add up to anything meaningful, and a betrayed goblin stack's going to die in one hit from just about anything. All this really means, at best, is you get to enjoy Witch Doctor/Trapper abilities for an extra turn.
I briefly considered Shak'Karukat before realizing the wyvern he starts with is actually quite a boon against early neutral stacks (depending on the map).
Inferno:
Best-- This probably won't come as a great surprise, but Deleb. Inferno struggles more than anyone with the beginning stages of the game, unless of course, they have arrows with fireballs stapled to them. Her ability and Triple Ballista lets her outpace damn near everyone.
Once again, props to Grok as resident Logistics hero. He does his job well, but loses just as many imps as any other Demon Lord. Marbas and Nymus can both be great at higher levels, but their talents are useless for the first stretch of the game.
Worst-- Bit of a toss up, but I'm going with Grawl. The starting Hellhounds are nice, but the only think that really makes the unit "good" per se is the No Retaliation they get on upgrading, and his bonus doesn't keep them from being a fragile target at any level. Starting with Eldritch Arrow is a nice touch if he has enough mana, but otherwise starting with Destruction Magic is nothing notable.
Alastor's not much better though, if at all. His bonus is kind of neat, but very limited in application. I only consider him marginally better because he starts with Mana Regeneration.
Haven
Best-- Maeve. If you can get your hands on Mass Haste, you're taking it either way. Why not capitalize on it? Once again, a hero who can leap into taking Retribution early on, and guaranteed to get the Leadership ranks to support it. Part of my reasoning is simply that having Maeve guarantees you have Haste, since you might otherwise be stuck with Divine Strength. If you end up getting Divine Strength anyway, guess who has two level 1 Light Magic spells!
While unlikely, Empathy + Mass Haste Maeve + Divine Guidance + >=5 Morale = Your enemies no longer get to take turns. Maeve takes a bit to really get going so she's not going to be fantastic on smaller maps, but I love this combo too much not to point it out.
And of course, Rutger, for going zooooooom. And as a faction that's a bit better at starting out, he's significantly more viable than Grok. While I don't think Haven needs as much of an early game advantage, Dougal can be pretty snazzy, too.
Worst-- This is a tough one, but I'm gonna say Laszlo. I don't have a lot of reason for this, it's actually kind of neat to start out with Footmen since you don't need to be as worried about early ranged neutrals like Gremlins, he just seems otherwise completely unremarkable compared to the rest.
Necropolis:
Best-- ...I really have no idea. I've never been able to decide. I tend to favor Vladmir, partly for the strong start Master of Life gives him despite how weak his bonus is, but mostly for his portrait. Deirdre's nice but I tend to find better things to use a hero turn for than Banshee's Scream anyway, and while she has very little to start her off, Lucretia's Sorcery start and Vampire bonus can be great in the long run. So... I'unno.
Worst-- Raven. The added effect to curse of weakness is definitely a welcome improvement, I've just never thought of Weakness as a go-to spell when you have Slow/Raise Dead/Puppet Master to throw around, and Destructive Magic is a wasted skill in Necropolis.
And that's all I have the attention span to write, right now. Let me just end by saying
Svea is useless, useless, useless. Why would you ever pick her. Why would you even use lightning magic. You could have Helmar or Brand, but no. You wanted to be able to cast the worst Destructive spell on the, what, two units immune to it?
Why does she exist. Why is she even a thing. Grawrglefargle.