The Wizard holds my answer to each of the above questions. I think Arcane Orb will be the most bang for your buck skill available. When you combine all factors (damage, cost, AoE, cast time, ability to spam) I have a strong feeling it'll eventually be nerfed.
As much fun as the skill seems, I find Hydra to be the most ineffective. It's a mere 20-30% weapon damage that's not guaranteed damage (random projectiles) and ability to only have one hydra active at a time. I understand the usefulness of being able to have that on top of your other main damaging skills, for example-casting ahead of you while running, but considering you only have six spell slots I just don't see myself using it. I'd rather have the permanent 20% weapon damage with other skill rune buffs provided by your familiar.
Without a doubt I'm most looking forward to using Teleport with Wormhole. It's one of those spells that defines the class. I'm a freaking Wizard and I can teleport! :P
I'd like to close with asking others not to criticize other people's answers too much, since we're all just speculating. For instance I have a feeling I'll be dead wrong about Hydra since I don't know how all of the skill runes will act, but that's how I feel based on my limited first hand knowledge.
I've only played the open beta, but I played through with the Witch Doctor, Wizard, Demon Hunter and the Monk in that order. Most of my opinion will be based on that.
1. Most OP of the beta for me was the Wizards Shock Pulse with Explosive Bolts. Once I got that I didn't need any other skill save for bosses or uniques. I felt Arcane Orb was too slow (I missed a lot) and the enemies were not packed close enough to be fully effective. Frost Ray took out the same pack without using all of my AP. I only used Arcane Orb if a unique was chasing me since I didn't have to stand still to cast it.
2. Worst looking skill to me is the Witch Doctor Plague of Toads with Toad of Hugeness. That skill is now supposed to be his spammable main attack. The Toad of Hugeness takes out 1 enemy, and just sits there. That's worthless. With the un-runed skill you could take out many more in the same amount of time. I would be surprised if that skill makes it into the game. I didn't feel any beta skill was worthless, just not my flavor.
3. The Wizard's Archon looks so epic. Who wouldn't look forward to that?
Well, I've been paying most attention to witch doctor.
WD's Soul Harvest has to be right up there for OP, unless it turns out it doesn't scale well. Massively increases damage for any other skill.
As for "fun", I am looking forward to Fetish Army Probably with the rune that decreases the cooldown, if I like using it as much as I expect to.
And for "dud", I'm gonna nominate Hex. It's a CC skill but pretty much only deals with one enemy at a time, whereas it competes with AoE CC skills. It's possible it will become viable when runed to do AoE damage as well though.
I've only really looked at the wizard, as that's my plan for the first character
Dud: I can see uses for most skills/runes, but there are a few I just don't get:
- Teleport (Fracture) - well, there isn't enough information on these decoys, but I don't see any reason to choose it over other runes
- Wave of Force (Teleporting Wave) - huh? Maybe it makes sense in-game somehow...
- Ice Armor (Frozen Storm) - not useless, but all other runes seem preferable
- Explosive Blast - just...don't get it.
- Mirror Image - not completely useless, but not worth it unless perhaps using Illusionist
- Paralysis passive - 5% chance IF you do lightning damage? Meh.
- Archon looks amazing. but also OP probably.
- Arcane Torrent (death blossom). Ok, perhaps few opportunities where it would be optimal, but when it is...Wow. Been trying to make a build around this.
- Disintegrate (I think the Median mod for D2 had a similar skill?)