So my choices here are based entirely on either being the Overlord or playing Road to Legend - I've not been a hero outside of RtL. And I've got all the expansions/characters that are available outside of 1st edition as well.
Ispher - Their ability is always useful. Always. They also manage to play some of the more esoteric healer classes without a problem because they don't really need to heal themselves. This is who I always pick to play when I'm being a hero.
Elder Mok - Said a billion times but Mok's ability is glorious and synergises really well with being a bard. He's just rock solid all around. Nice stats too.
Lindel - The app has a lot of tests to do. A LOT of tests, far more than you'd ever do in a normal game. Lindel can make a lot of them much easier. This especially applies to areas where the game is sort of pushing a "warrior carries this item, scout carries this, mage carries this" because Lindel can just carry anything and do well with it.
Okaluk and Rakash - The most mobile hero in the game. Being able to move 4 without wasting an action is just insanely good. Especially when you want to open a door and move away, which happens all the time.
Sir Valadir - He's got a heroic feat that's actually useful in the app, which is actually quite uncommon. And his surges mean things like Skirmisher is a really good choice and can remove a lot of randomness.
Honourable mention goes to One Fist. He's super situational though and you have to kinda build around him which isn't the case with the other heroes.
Best Classes: Beastmaster - Has a strong pet and a lot of passive abilities which is really important when it comes to forgetting stuff.
Bard - In the app group healing is king and the bard is the best at it.
Skirmisher - Can boost their stamina and also gets a ton of attacks all on the same target, really good for taking down very strong single lieutenants and others. Killing is more important in the app than the normal game, so they become much more useful and flexible. Also goes super well with the hybrid mage.
Battlemage - With the exception of Runemaster all the mage classes are rubbish and being able to drop having a mage is great. Again that might not apply to playing normally but in the app it's definitely true. Adding this class to Jaes the Exile and using a bunch of runes (which often come up in the shop and are wasted) makes it a lot of fun too, so much variety.
Shadow Warrior - Being able to add hearts to people as well as generally boost attacks is just unbelievably handy in the app. Doing more damage is all you need to do, killing more monsters or keeping them out of the way is just extremely important.
Worst Classes: Every Mage and Scout Class - I'm discounting Runemaster and Shadow Warrior here. Treasure Hunter is great when you're playing normally and need those search cards. In the app search cards are almost meaningless which really shoots the TH in the foot. Thief is a weaker version of the TH already so that goes out of the window. Wildlander is very much a one trick pony and doesn't work as well in the app as it does in the real game.
The mages that summon things to mess around are mostly just in the way because in the app doing loads of damage is way more important.
Marshal - As an Overlord Marshal makes my game really tedious and in the app the Marshal doesn't do anything at all. I dunno if I've ever had someone play it.
Spiritspeaker - It can't heal statuses. Every single healer class works better than this one, even Prophet which is already quite fiddly and annoying. And their main ability is "before dice are rolled" which makes it even more terrible!
This is a harder one for me because I don't really remember the names of most of the heroes since they never get used. Anyone with a brown defence dice is ignored because you're going to just get walloped really early. Ones based around rez abilities are meh because you don't want to die since you've only got 5 deaths before you lose the game. I've got every hero but we only regularly use maybe....15 of them? And even then some are for gimmick parties like "all orcs" - which is hard because there's only one Orc scout and she's really not very good!
I'll give an honourable mention to Steelhorns and the....Yeti hero. They both look unbelievably good but their abilities are terrible and I just can't justify ever using them.